Add textureGather and textureGatherOffset 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>