Use kLow_SkFilterQuality when generating shader masks

This prevents edge aliasing for kDecal tiling.

Change-Id: I3a66e49ec1900bbe19c6700ec2f766122af4a3ce
Reviewed-on: https://skia-review.googlesource.com/113124
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>

src/effects/SkShaderMaskFilter.cpp

@@ -103,6 +103,7 @@ bool SkShaderMF::filterMask(SkMask* dst, const SkMask& src, const SkMatrix& ctm,
 
     SkPaint paint;
     paint.setShader(fShader);
+    paint.setFilterQuality(SkFilterQuality::kLow_SkFilterQuality);
     // this blendmode is the trick: we only draw the shader where the mask is
     paint.setBlendMode(SkBlendMode::kSrcIn);