Provide a GPU implementation of SkArithmeticMode, using a custom GrEffect exposed...

Provide a GPU implementation of SkArithmeticMode, using a custom GrEffect exposed via asNewEffectOrCoeff().

Doing it this way required modifying the arithmode GM to use saveLayer()/restore() rather than creating an offscreen SkBitmap, since otherwise the compositing is always done in raster mode. Fixing that in turn exposed that SkArithmeticMode did not work in Picture mode, since it wasn't flattenable. Made it so.

Note: this will require rebaselining the arithmode GM (again).

R=bsalomon@google.com, reed@google.com

Originally committed: https://code.google.com/p/skia/source/detail?r=9324
Reverted: https://code.google.com/p/skia/source/detail?r=9325

Review URL: https://codereview.chromium.org/16064002

git-svn-id: http://skia.googlecode.com/svn/trunk@9330 2bbb7eff-a529-9590-31e7-b0007b416f81