Is much faster for completeness cache checks in syncProgramTextures.

Bug: angleproject:2763
Change-Id: Iffdacbb8a4f6640caa5051643c379a7b4c3311b6
Reviewed-on: https://chromium-review.googlesource.com/1171508
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>