Fix Normal() not factoring in opacity

The alpha calculation in Normal() did not factor in opacity, causing
layer and cel opacity to render incorrectly against transparent layers.

Author: guptohkl

source/AsepriteDotNet/Aseprite/AsepriteColorUtilities.cs

@@ -245,7 +245,7 @@ private static Rgba32 Normal(Rgba32 backdrop, Rgba32 source, int opacity)
 
         opacity = Calc.MultiplyUnsigned8Bit(source.A, opacity);
 
-        int a = source.A + backdrop.A - Calc.MultiplyUnsigned8Bit(backdrop.A, source.A);
+        int a = backdrop.A + opacity - Calc.MultiplyUnsigned8Bit(backdrop.A, opacity);
         int r = backdrop.R + (source.R - backdrop.R) * opacity / a;
         int g = backdrop.G + (source.G - backdrop.G) * opacity / a;
         int b = backdrop.B + (source.B - backdrop.B) * opacity / a;