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

src/core/SkBlitter_A8.cpp

Index: src/core/SkBlitter_A8.cpp
diff --git a/src/core/SkBlitter_A8.cpp b/src/core/SkBlitter_A8.cpp
index 57587c675a7197b1b069f51fd535443bc129abcb..66976143c8f3df7fe9e8fdd5496c9e99ec02c9a5 100644
--- a/src/core/SkBlitter_A8.cpp
+++ b/src/core/SkBlitter_A8.cpp
@@ -276,7 +276,7 @@ static inline uint8_t aa_blend8(SkPMColor src, U8CPU da, int aa) {
 
     int src_scale = SkAlpha255To256(aa);
     int sa = SkGetPackedA32(src);
-    int dst_scale = 256 - SkAlphaMul(sa, src_scale);
+    int dst_scale = SkAlphaMulInv256(sa, src_scale);
 
     return SkToU8((sa * src_scale + da * dst_scale) >> 8);
 }