trying to improve Opacity out of range propagation.

For some reason, FillRegionProcessor and DrawTextProcessor where overshooting opacity over 1

src/ImageSharp.Drawing/Processing/Processors/Drawing/FillRegionProcessor.cs

@@ -139,9 +139,11 @@ namespace SixLabors.ImageSharp.Processing.Processors.Drawing
                                 {
                                     for (float x = scanStart; x < startX + 1; x += subpixelFraction)
                                     {
-                                        scanline[startX] += subpixelFractionPoint;
-                                        scanlineDirty = true;
-                                    }
+                                        scanline[startX] += subpixelFractionPoint;
+                                        scanlineDirty = true;
+                                    }
+
+                                    if (scanline[startX] > 1) { scanline[startX] = 1; }
                                 }
 
                                 if (endX >= 0 && endX < scanline.Length)
@@ -149,8 +151,10 @@ namespace SixLabors.ImageSharp.Processing.Processors.Drawing
                                     for (float x = endX; x < scanEnd; x += subpixelFraction)
                                     {
                                         scanline[endX] += subpixelFractionPoint;
-                                        scanlineDirty = true;
-                                    }
+                                        scanlineDirty = true;
+                                    }
+
+                                    if (scanline[endX] > 1) { scanline[endX] = 1; }
                                 }
 
                                 int nextX = startX + 1;
@@ -158,8 +162,10 @@ namespace SixLabors.ImageSharp.Processing.Processors.Drawing
                                 nextX = Math.Max(nextX, 0);
                                 for (int x = nextX; x < endX; x++)
                                 {
-                                    scanline[x] += subpixelFraction;
-                                    scanlineDirty = true;
+                                    scanline[x] += subpixelFraction;
+                                    scanlineDirty = true;
+
+                                    if (scanline[x] > 1) { scanline[x] = 1; }
                                 }
                             }
                         }

src/ImageSharp.Drawing/Processing/Processors/Text/DrawTextProcessor.cs

@@ -386,7 +386,9 @@ namespace SixLabors.ImageSharp.Processing.Processors.Text
                                     {
                                         scanline[startX] += subpixelFractionPoint;
                                         scanlineDirty = true;
-                                    }
+                                    }
+
+                                    if (scanline[startX] > 1) { scanline[startX] = 1; }
                                 }
 
                                 if (endX >= 0 && endX < scanline.Length)
@@ -395,7 +397,9 @@ namespace SixLabors.ImageSharp.Processing.Processors.Text
                                     {
                                         scanline[endX] += subpixelFractionPoint;
                                         scanlineDirty = true;
-                                    }
+                                    }
+
+                                    if (scanline[endX] > 1) { scanline[endX] = 1; }
                                 }
 
                                 int nextX = startX + 1;
@@ -404,7 +408,9 @@ namespace SixLabors.ImageSharp.Processing.Processors.Text
                                 for (int x = nextX; x < endX; x++)
                                 {
                                     scanline[x] += subpixelFraction;
-                                    scanlineDirty = true;
+                                    scanlineDirty = true;
+
+                                    if (scanline[x] > 1) { scanline[x] = 1; }
                                 }
                             }
                         }

src/ImageSharp.Drawing/Processing/SolidBrush{TPixel}.cs

@@ -91,9 +91,18 @@ namespace SixLabors.ImageSharp.Processing
             {
                 Span<TPixel> destinationRow = this.Target.GetPixelRowSpan(y).Slice(x, scanline.Length);
 
-                MemoryAllocator memoryAllocator = this.Target.MemoryAllocator;
+                MemoryAllocator memoryAllocator = this.Target.MemoryAllocator;
+
+#if DEBUG
+                for (int i = 0; i < scanline.Length; i++)
+                {
+                    Guard.MustBeBetweenOrEqualTo(scanline[i], 0, 1, nameof(scanline));
+                }
+#endif
+
+                float opacity = this.Options.BlendPercentage.Clamp(0, 1);
 
-                if (this.Options.BlendPercentage == 1f)
+                if (opacity == 1f)
                 {
                     this.Blender.Blend(memoryAllocator, destinationRow, destinationRow, this.Colors.GetSpan(), scanline);
                 }
@@ -105,7 +114,7 @@ namespace SixLabors.ImageSharp.Processing
 
                         for (int i = 0; i < scanline.Length; i++)
                         {
-                            amountSpan[i] = scanline[i] * this.Options.BlendPercentage;
+                            amountSpan[i] = scanline[i] * opacity;
                         }
 
                         this.Blender.Blend(memoryAllocator, destinationRow, destinationRow, this.Colors.GetSpan(), amountSpan);