Check that texture offset is constant and valid Offset passed to textureOffset and similar functions must be constant. See ESSL 3.00 spec section 8.8. It must also be in the valid range between MIN_PROGRAM_TEXEL_OFFSET and MAX_PROGRAM_TEXEL_OFFSET. Using values outside the valid range makes the results of the texture lookup undefined, as specified in GLES 3.0.4 section 3.8.10. We generate a compiler error if an offset is outside the valid range. BUG=angleproject:1215 TEST=angle_unittests Change-Id: Ida28361444d2f4050d516160f1491674c31868a1 Reviewed-on: https://chromium-review.googlesource.com/312223 Reviewed-by: Jamie Madill <jmadill@chromium.org> Tested-by: Olli Etuaho <oetuaho@nvidia.com>