fix fillRect with alpha image

Author: brendan-duncan

lib/src/draw/fill_rect.dart

@@ -103,11 +103,12 @@ Image fillRect(Image src,
     while (iter.moveNext()) {
       final p = iter.current;
       final m = mask?.getPixel(p.x, p.y).getChannelNormalized(maskChannel) ?? 1;
+      final am = color.a * m;
       p
         ..r = mix(p.r, color.r, a * m)
         ..g = mix(p.g, color.g, a * m)
         ..b = mix(p.b, color.b, a * m)
-        ..a = p.a * (1 - (color.a * m));
+        ..a = p.a * (1 - am) + am;
     }
   }
 

test/draw/fill_rect_test.dart

@@ -35,7 +35,7 @@ void main() {
       expect(p, equals([255, 0, 0, 255]));
 
       p = i0.getPixel(195, 195);
-      expect(p, equals([0, 128, 0, 0]));
+      expect(p, equals([0, 128, 0, 128]));
 
       final mask = Command()
         ..createImage(width: 256, height: 256)