A user may be using Program Pipelines, rather than monolithic Programs,
so State should check if mExecutable is valid, rather than mProgram,
since that indicates the presence of either a PPO or a Program.

Exercising these paths requires additional tests:
SimpleStateChangeTestComputeES31PPO::DeleteImageTextureInUse()
Texture2DTestES31PPO::TexStorage()
Texture2DTestES31PPO::SingleTextureMultipleSamplers()

These new tests exposed bugs in the PPO implementation where updates to
the active Program's ProgramExecutable were not being propagated to the
Executables of the PPO's containing that Program. In these particular
cases, updates to the active samplers/images/textures were not being
copied to the PPO's Executable.

Bug: angleproject:3570
Test: end2end tests listed above
Change-Id: I297cac2d0367f180dd7fa01a1ee7ba53996867c4
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2225417
Commit-Queue: Tim Van Patten <timvp@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>