This CL adds support for a Program Pipeline Object to have a VS, FS,
and CS attached to the same PPO and then using that PPO for both draw
and dispatch calls.

Bug: angleproject:3570
Test: KHR-GLES31.core.compute_shader.sso*
Change-Id: I262cdbdfd442f6db5ba2b45d1308003102b237cb
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2150078Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Commit-Queue: Tim Van Patten <timvp@google.com>