Updating sampler uniforms when using PPOs is currently broken, since the
Context/State use the currently bound ProgramExecutable which belongs to
the PPO, but the updates only happen to the Program's executable that
the uniform belongs to.

This change updates Program::updateSamplerUniform() to update any PPO
ProgramExecutables with the updated texture information when a Program's
sampler uniforms are updated, so the Context/State use the correct data.

Bug: b/182409935
Test: ProgramPipelineTest31.SampleTextureAThenTextureB
Test: SamplersTest31.SampleTextureAThenTextureB
Change-Id: I3c4e156c6e0c781e706f321f0bd12baf484ff42a
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2797951
Commit-Queue: Tim Van Patten <timvp@google.com>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>