fix span length issues related to Vector4 conversion

src/ImageSharp/PixelFormats/PixelBlender{TPixel}.cs

@@ -93,12 +93,12 @@ namespace SixLabors.ImageSharp.PixelFormats
                 Span<Vector4> backgroundSpan = buffer.Slice(destination.Length, destination.Length);
                 Span<Vector4> sourceSpan = buffer.Slice(destination.Length * 2, destination.Length);
 
-                PixelOperations<TPixel>.Instance.ToScaledVector4(background, backgroundSpan);
-                PixelOperations<TPixelSrc>.Instance.ToScaledVector4(source, sourceSpan);
+                PixelOperations<TPixel>.Instance.ToScaledVector4(background.Slice(0, background.Length), backgroundSpan);
+                PixelOperations<TPixelSrc>.Instance.ToScaledVector4(source.Slice(0, background.Length), sourceSpan);
 
                 this.BlendFunction(destinationSpan, backgroundSpan, sourceSpan, amount);
 
-                PixelOperations<TPixel>.Instance.FromScaledVector4(destinationSpan, destination);
+                PixelOperations<TPixel>.Instance.FromScaledVector4(destinationSpan.Slice(0, background.Length), destination);
             }
         }
 
@@ -127,12 +127,12 @@ namespace SixLabors.ImageSharp.PixelFormats
                 Span<Vector4> backgroundSpan = buffer.Slice(destination.Length, destination.Length);
                 Span<Vector4> sourceSpan = buffer.Slice(destination.Length * 2, destination.Length);
 
-                PixelOperations<TPixel>.Instance.ToScaledVector4(background, backgroundSpan);
-                PixelOperations<TPixelSrc>.Instance.ToScaledVector4(source, sourceSpan);
+                PixelOperations<TPixel>.Instance.ToScaledVector4(background.Slice(0, background.Length), backgroundSpan);
+                PixelOperations<TPixelSrc>.Instance.ToScaledVector4(source.Slice(0, background.Length), sourceSpan);
 
                 this.BlendFunction(destinationSpan, backgroundSpan, sourceSpan, amount);
 
-                PixelOperations<TPixel>.Instance.FromScaledVector4(destinationSpan, destination);
+                PixelOperations<TPixel>.Instance.FromScaledVector4(destinationSpan.Slice(0, background.Length), destination);
             }
         }
     }

src/ImageSharp/Processing/Processors/Convolution/Convolution2DProcessor.cs

@@ -75,14 +75,14 @@ namespace SixLabors.ImageSharp.Processing.Processors.Convolution
                             for (int y = rows.Min; y < rows.Max; y++)
                             {
                                 Span<TPixel> targetRowSpan = targetPixels.GetRowSpan(y).Slice(startX);
-                                PixelOperations<TPixel>.Instance.ToVector4(targetRowSpan, vectorSpan);
+                                PixelOperations<TPixel>.Instance.ToVector4(targetRowSpan.Slice(0, length), vectorSpan);
 
                                 for (int x = 0; x < width; x++)
                                 {
                                     DenseMatrixUtils.Convolve2D(in matrixY, in matrixX, source.PixelBuffer, vectorSpan, y, x, maxY, maxX, startX);
                                 }
 
-                                PixelOperations<TPixel>.Instance.FromVector4(vectorSpan, targetRowSpan);
+                                PixelOperations<TPixel>.Instance.FromVector4(vectorSpan.Slice(0, length), targetRowSpan);
                             }
                         });
 

src/ImageSharp/Processing/Processors/Convolution/Convolution2PassProcessor.cs

@@ -93,14 +93,14 @@ namespace SixLabors.ImageSharp.Processing.Processors.Convolution
                         for (int y = rows.Min; y < rows.Max; y++)
                         {
                             Span<TPixel> targetRowSpan = targetPixels.GetRowSpan(y).Slice(startX);
-                            PixelOperations<TPixel>.Instance.ToVector4(targetRowSpan, vectorSpan);
+                            PixelOperations<TPixel>.Instance.ToVector4(targetRowSpan.Slice(0, length), vectorSpan);
 
                             for (int x = 0; x < width; x++)
                             {
                                 DenseMatrixUtils.Convolve(in matrix, sourcePixels, vectorSpan, y, x, maxY, maxX, startX);
                             }
 
-                            PixelOperations<TPixel>.Instance.FromVector4(vectorSpan, targetRowSpan);
+                            PixelOperations<TPixel>.Instance.FromVector4(vectorSpan.Slice(0, length), targetRowSpan);
                         }
                     });
         }

src/ImageSharp/Processing/Processors/Convolution/ConvolutionProcessor.cs

@@ -59,14 +59,14 @@ namespace SixLabors.ImageSharp.Processing.Processors.Convolution
                             for (int y = rows.Min; y < rows.Max; y++)
                             {
                                 Span<TPixel> targetRowSpan = targetPixels.GetRowSpan(y).Slice(startX);
-                                PixelOperations<TPixel>.Instance.ToVector4(targetRowSpan, vectorSpan);
+                                PixelOperations<TPixel>.Instance.ToVector4(targetRowSpan.Slice(0, length), vectorSpan);
 
                                 for (int x = 0; x < width; x++)
                                 {
                                     DenseMatrixUtils.Convolve(in matrix, source.PixelBuffer, vectorSpan, y, x, maxY, maxX, startX);
                                 }
 
-                                PixelOperations<TPixel>.Instance.FromVector4(vectorSpan, targetRowSpan);
+                                PixelOperations<TPixel>.Instance.FromVector4(vectorSpan.Slice(0, length), targetRowSpan);
                             }
                         });
 

src/ImageSharp/Processing/Processors/Quantization/FrameQuantizerBase{TPixel}.cs

@@ -63,7 +63,7 @@ namespace SixLabors.ImageSharp.Processing.Processors.Quantization
         public virtual QuantizedFrame<TPixel> QuantizeFrame(ImageFrame<TPixel> image)
         {
             Guard.NotNull(image, nameof(image));
-
+            
             // Get the size of the source image
             int height = image.Height;
             int width = image.Width;

tests/ImageSharp.Tests/Issues/Issue412.cs

@@ -12,7 +12,7 @@ namespace SixLabors.ImageSharp.Tests.Issues
         [WithBlankImages(40, 30, PixelTypes.Rgba32)]
         public void AllPixelsExpectedToBeRedWhenAntialiasedDisabled<TPixel>(TestImageProvider<TPixel> provider) where TPixel : struct, IPixel<TPixel>
         {
-            using (var image = provider.GetImage())
+            using (Image<TPixel> image = provider.GetImage())
             {
                 image.Mutate(
                     context =>