"Command & Conquer: Rivals" uses PPOs and was hitting the following
assert:

    angle::Result ContextVk::updateActiveTextures(const gl::Context *context)
    {
    ...
        for (size_t textureUnit : activeTextures)
        {
            gl::Texture *texture        = textures[textureUnit];
            gl::TextureType textureType = textureTypes[textureUnit];
            ASSERT(textureType != gl::TextureType::InvalidEnum);

This is the same assert that is generated by the test
ProgramPipelineTest31.DifferentTextureTypes which is currently being
skipped since it's known to fail.

This CL refactors sampler validation into the ProgramExecutable to allow
PPOs to take advantage of the shared code and behave correctly, since
the necessary data is already copied into the PPO's ProgramExecutable
via ProgramExecutable::updateActiveSamplers(). This also takes advantage
of the subject/observer pattern that's already established between
programs and PPOs to ensure only the PPOs that the program is a part of
are updated when a program's sampler uniforms are updated.

Bug: angleproject:3570
Bug: b/182409935
Test: ProgramPipelineTest31.DifferentTextureTypes
Change-Id: I3d34efd66dc85e7ff23a8422cb14d5f90a5f7085
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2792862
Commit-Queue: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>