Mark texture2D() functions as not having side effects. This should allow the shader translator to remove unused variables and cut out sampling instructions in apps like T-Rex. Bug: angleproject:6061 Change-Id: If543c9c737cdbcfdea1433876cd425e7335e5297 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3862383 Reviewed-by: Charlie Lao <cclao@google.com> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>