Add 8888 fast SrcOver mode.

src/core/SkLinearBitmapPipeline_tile.h

 /*
  * Copyright 2016 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkLinearBitmapPipeline_tile_DEFINED
 #define SkLinearBitmapPipeline_tile_DEFINED
 
@@ skipping 280 matching lines @@
         // Note: The span length has an unintuitive relation to the tile width. The tile width is
         // a half open interval [tb, te), but the span is a closed interval [sb, se]. In order to
         // compare the two, you need to convert the span to a half open interval. This is done by
         // adding dx to se. So, the span becomes: [sb, se + dx). Hence the + 1.0f below.
         SkScalar div = (span.length() + 1.0f) / fXMax;
         int32_t repeatCount = SkScalarFloorToInt(div);
         Span repeatableSpan{{0.0f, y}, fXMax - 1.0f, SkScalarFloorToInt(fXMax)};
 
         // 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;
 
         // There may be some of the span left over.
         span.breakAt(advance, 1.0f);
 
         // All on a single tile.
         if (!span.isEmpty()) {
             span.offset(-advance);
@@ skipping 101 matching lines @@
 
 private:
     SkScalar fYMax;
     Sk4f     fYsMax;
     Sk4f     fYsCap;
     Sk4f     fYsDoubleInvMax;
 };
 
 }  // namespace
 #endif  // SkLinearBitmapPipeline_tile_DEFINED