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/312223Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>