revise row blits to keep intermediate precision so that color is preserved when blended against its…

src/core/SkBlitter_ARGB32.cpp

Index: src/core/SkBlitter_ARGB32.cpp
diff --git a/src/core/SkBlitter_ARGB32.cpp b/src/core/SkBlitter_ARGB32.cpp
index 7adab5544537fd052897d4dfdcb3e4a6b20246b4..aada0586a4b2dfb0bd7e2e9ae221b613b41138d1 100644
--- a/src/core/SkBlitter_ARGB32.cpp
+++ b/src/core/SkBlitter_ARGB32.cpp
@@ -237,7 +237,11 @@ void SkARGB32_Blitter::blitV(int x, int y, int height, SkAlpha alpha) {
         color = SkAlphaMulQ(color, SkAlpha255To256(alpha));
     }
 
+#ifdef SK_SUPPORT_LEGACY_BROKEN_LERP
     unsigned dst_scale = 255 - SkGetPackedA32(color);
+#else
+    unsigned dst_scale = SkAlpha255To256(255 - SkGetPackedA32(color));
+#endif
     size_t rowBytes = fDevice.rowBytes();
     while (--height >= 0) {
         device[0] = color + SkAlphaMulQ(device[0], dst_scale);