This mask cleans up some of the iteration logic in
State::syncProgramTextures. It will make it easier to optimize this
function in the future. This will also make it easier to recompute
the sampler type validation.

Leads to a 5% improvement in State::syncProgramTextures.

Bug: angleproject:2747
Bug: angleproject:2763
Change-Id: Ic9a555df843ad23b4c562e6e4a2d425bee58a856
Reviewed-on: https://chromium-review.googlesource.com/1164306
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>