Add 8888 fast SrcOver mode.

src/core/SkLinearBitmapPipeline_tile.h

Index: src/core/SkLinearBitmapPipeline_tile.h
diff --git a/src/core/SkLinearBitmapPipeline_tile.h b/src/core/SkLinearBitmapPipeline_tile.h
index 997ab65c4730cd11dd475edb96c8ad6b6d584859..6fe64f1bce4c5ffd3dd5c8ee1f598c0332320e5f 100644
--- a/src/core/SkLinearBitmapPipeline_tile.h
+++ b/src/core/SkLinearBitmapPipeline_tile.h
@@ -298,7 +298,9 @@ public:
 
         // Repeat the center section.
         SkASSERT(0.0f <= repeatableSpan.startX() && repeatableSpan.endX() < fXMax);
-        next->repeatSpan(repeatableSpan, repeatCount);
+        if (repeatCount > 0) {
+            next->repeatSpan(repeatableSpan, repeatCount);
+        }
 
         // Calculate the advance past the center portion.
         SkScalar advance = SkScalar(repeatCount) * fXMax;