Bug 1011062. Add a fast path to DrawTargetSkia::MaskSurface(). r=gw280

This should speed things up significantly by avoid a bunch
of unneeded work.

--HG--
extra : rebase_source : 1194a825a5a9e5e86a94ba6c6c67f959051f1da3

gfx/2d/DrawTargetSkia.cpp

@@ -552,6 +552,10 @@ DrawTargetSkia::MaskSurface(const Pattern &aSource,
   MarkChanged();
   AutoPaintSetup paint(mCanvas.get(), aOptions, aSource);
 
+  TempBitmap bitmap = GetBitmapForSurface(aMask);
+  if (bitmap.mBitmap.colorType() == kAlpha_8_SkColorType) {
+    mCanvas->drawBitmap(bitmap.mBitmap, aOffset.x, aOffset.y, &paint.mPaint);
+  } else {
     SkPaint maskPaint;
     TempBitmap tmpBitmap;
     SetPaintPattern(maskPaint, SurfacePattern(aMask, ExtendMode::CLAMP), tmpBitmap);
@@ -568,6 +572,7 @@ DrawTargetSkia::MaskSurface(const Pattern &aSource,
     Rect rect = Rect(aOffset.x, aOffset.y, size.width, size.height);
     mCanvas->drawRect(RectToSkRect(rect), paint.mPaint);
   }
+}
 
 TemporaryRef<SourceSurface>
 DrawTargetSkia::CreateSourceSurfaceFromData(unsigned char *aData,