Desktop OpenGL before 4.4 doesn't handle SRGB blits the same way OpenGL
ES does. Emulate them by drawing a quad.

BUG=angleproject:1492
BUG=chromium:634525

Change-Id: I9f2992d9b373941b10f19f8a51564f0f756cc4df
Reviewed-on: https://chromium-review.googlesource.com/389853Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>