Add discard API to SkCanvas, plumb it to glDiscardFramebuffer()


BUG=skia:2349
R=robertphillips@google.com, reed@google.com

Author: bsalomon@google.com

Review URL: https://codereview.chromium.org/211683002

git-svn-id: http://skia.googlecode.com/svn/trunk@13976 2bbb7eff-a529-9590-31e7-b0007b416f81
22 files changed