Caching the result of readPixelsSupported

The call was calling GR_GL_GetIntegerv 2 times for each readPixels
and thus was causing a loss of performance

(resubmit of issue 344793008)

Benchmark url: http://packages.gkny.fr/tst/index.html

BUG=skia:2681
R=junov@chromium.org

Author: piotaixr@chromium.org

Review URL: https://codereview.chromium.org/364193004
3 files changed