This code is analoguous to the code in TextureFunctionHLSL and is now
implemented in a similar manner.

BUG=angleproject:2267
TEST=angle_unittests, angle_end2end_tests

Change-Id: Ie3503766217dad4f3848f2d4b2fc3f62b3edce0c
Reviewed-on: https://chromium-review.googlesource.com/1110366Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>