Add missing fallback blending to SkA8_Shader_Blitter::blitMask()

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 02554643d26e4380e7760b0e6a0f62b2468c2244..7fca29ecf5d0c2e31c12b0fe9b01bdceba74d8e6 100644
--- a/src/core/SkBlitter_A8.cpp
+++ b/src/core/SkBlitter_A8.cpp
@@ -336,6 +336,10 @@ void SkA8_Shader_Blitter::blitMask(const SkMask& mask, const SkIRect& clip) {
         fShader->shadeSpan(x, y, span, width);
         if (fXfermode) {
             fXfermode->xferA8(device, span, width, alpha);
+        } else {
+            for (int i = width - 1; i >= 0; --i) {
+                device[i] = aa_blend8(span[i], device[i], alpha[i]);
+            }
         }
 
         y += 1;