Implement texelFetch and texelFetchOffset. BUG=angle:564 Change-Id: I7ad3484061f5d6912d7842bdadc6297de3e82ef8 Reviewed-on: https://chromium-review.googlesource.com/186990 Tested-by: Nicolas Capens <nicolascapens@chromium.org> Reviewed-by: Shannon Woods <shannonwoods@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org>