Add GPU support for roundrects

Add GPU support for roundrects

This uses the OvalRenderer to render roundrects as "stretched ovals." It adds an
additional shader that handles the straight edges of ellipsoid roundrects better,
and uses the circle shader for roundrects where the two radii are the same. Only
axis-aligned, simple roundrects are supported. Handles fill, stroke and hairline.

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

Author: jvanverth@google.com

Review URL: https://chromiumcodereview.appspot.com/13852049

git-svn-id: http://skia.googlecode.com/svn/trunk@8859 2bbb7eff-a529-9590-31e7-b0007b416f81