This CL updates the frontend to use the subject/observer pattern for
Programs and ProgramExecutables in PPOs.

This allows us to remove the hack that iterated through all PPOs and
marking them all dirty when a Program is re-linked. Instead, just the
PPOs the Program is bound to are signalled, and only when the Program
is in a PPO.

Additionally the necessary PPOs are signalled when an Executable's
textures are updated, rather than iterating through all PPOs and
signalling them.

Bug: angleproject:4559
Test: CQ
Change-Id: Iaefb88c64c13259e921c8dfe95cf640247d17f85
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2300705
Commit-Queue: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Charlie Lao <cclao@google.com>