fix slowness of getImageData() for CanvasRenderingContext2D in linux due to un-optimized format for glReadPixels

This is a continuation of https://codereview.chromium.org/15331003/

BUG=242093
R=robertphillips@google.com, jun.a.jiang@intel.com, bsalomon@google.com

Author: bsalomon@google.com

Review URL: https://chromiumcodereview.appspot.com/15746007

git-svn-id: http://skia.googlecode.com/svn/trunk@9280 2bbb7eff-a529-9590-31e7-b0007b416f81
4 files changed