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

@@ -142,6 +142,8 @@ namespace SixLabors.ImageSharp.Processing.Processors.Drawing
                                         scanline[startX] += subpixelFractionPoint;
                                         scanlineDirty = true;
                                     }
+
+                                    if (scanline[startX] > 1) { scanline[startX] = 1; }
                                 }
 
                                 if (endX >= 0 && endX < scanline.Length)
@@ -151,6 +153,8 @@ namespace SixLabors.ImageSharp.Processing.Processors.Drawing
                                         scanline[endX] += subpixelFractionPoint;
                                         scanlineDirty = true;
                                     }
+
+                                    if (scanline[endX] > 1) { scanline[endX] = 1; }
                                 }
 
                                 int nextX = startX + 1;
@@ -160,6 +164,8 @@ namespace SixLabors.ImageSharp.Processing.Processors.Drawing
                                 {
                                     scanline[x] += subpixelFraction;
                                     scanlineDirty = true;
+
+                                    if (scanline[x] > 1) { scanline[x] = 1; }
                                 }
                             }
                         }

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

@@ -387,6 +387,8 @@ namespace SixLabors.ImageSharp.Processing.Processors.Text
                                         scanline[startX] += subpixelFractionPoint;
                                         scanlineDirty = true;
                                     }
+
+                                    if (scanline[startX] > 1) { scanline[startX] = 1; }
                                 }
 
                                 if (endX >= 0 && endX < scanline.Length)
@@ -396,6 +398,8 @@ namespace SixLabors.ImageSharp.Processing.Processors.Text
                                         scanline[endX] += subpixelFractionPoint;
                                         scanlineDirty = true;
                                     }
+
+                                    if (scanline[endX] > 1) { scanline[endX] = 1; }
                                 }
 
                                 int nextX = startX + 1;
@@ -405,6 +409,8 @@ namespace SixLabors.ImageSharp.Processing.Processors.Text
                                 {
                                     scanline[x] += subpixelFraction;
                                     scanlineDirty = true;
+
+                                    if (scanline[x] > 1) { scanline[x] = 1; }
                                 }
                             }
                         }

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

@@ -93,7 +93,16 @@ namespace SixLabors.ImageSharp.Processing
 
                 MemoryAllocator memoryAllocator = this.Target.MemoryAllocator;
 
-                if (this.Options.BlendPercentage == 1f)
+#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 (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);