Removes the concept of the program textures dirty object. Instead we
use a set of dirty bits to represent dirty texture samples. We mark
certain textures dirty and update state structures whenever there is
a new Texture/Program/Sampler bound, or when Texture/Program/Sampler
state changes.

This is in preparation for making clearing the uncleared active
textures into a dirty bit as well.

Also includes new dirty bit handling for texture image units. These are
a GLES 3.1 feature.

Bug: angleproject:2966
Change-Id: Ibb8619dd2669bb39fdbcd75e3685be9a8aeeee91
Reviewed-on: https://chromium-review.googlesource.com/c/1346649
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>