The member ProgramState::mImageBindings is being moved to
ProgramExecutable to allow ProgramExecutable::getImageBindings() to
answer the query without relying on the Program[Pipeline]State.

Bug: angleproject:4520
Test: Build/CQ
Change-Id: Ia4934c8e57b5ba49b0a399dcad7c1b324c43385d
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2241750Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Tim Van Patten <timvp@google.com>