Improves performance on the render-to-texture microbenchmark
by ~3x on the OpenGL back-end. Wipes out several of the top profling
hotspots on that benchmark.

BUG=angleproject:1388

Change-Id: I6a35a0b435b2ed3c83d32acdb9df090df98214ad
Reviewed-on: https://chromium-review.googlesource.com/348957Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>