Caching the result of readPixelsSupported

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

Committed: https://skia.googlesource.com/skia/+/753a2964afe5661ce9b2a8ca77ca9d0aabd3173c

R=junov@chromium.org, reed@chromium.org, bsalomon@chromium.org, mtklein@google.com, bsalomon@google.com

Author: piotaixr@chromium.org

Review URL: https://codereview.chromium.org/364193004