Move alpha-ramp AA to GrContext, detect cases when AA is applied via other methods...

Move alpha-ramp AA to GrContext, detect cases when AA is applied via other methods (smooth lines, MSAA) or rect falls on integer coords and skip the alpha ramp path. Use pre-fab index buffer for alpha-ramped fill rects and stroke rects.

Review URL: http://codereview.appspot.com/4449047/

git-svn-id: http://skia.googlecode.com/svn/trunk@1169 2bbb7eff-a529-9590-31e7-b0007b416f81