The active samplers and images are being moved from Program into
ProgramExecutable to unify interacting with them for Programs and
ProgramPipelines

Also, create some helper functions for gl::Program that ProgramExecutable can
call to make it easier for ProgramPipeline to respond to similar
queries for each of the Programs in the ProgramPipeline.

Bug: angleproject:3570
Change-Id: I0b37f1a379e56b9659d82d92f6d7a546beee11cd
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2087648
Commit-Queue: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>