The patch adds new built-ins and extends the semantic parser to add
support for textureGather and textureGatherOffset.
BUG=angleproject:1442
TEST=angle_unittests
TEST=angle_deqp_gles31_tests.exe
--deqp-case=dEQP-GLES31.functional.texture.gather*
--deqp-egl-display-type=angle-gl
Change-Id: Iaf98c3420fbd61193072fdec8f5a61ac4c574101
Reviewed-on: https://chromium-review.googlesource.com/660124
Commit-Queue: Martin Radev <mradev@nvidia.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
