PipelineDesc describes a Vertex-Fragment pipeline and PipelineCache (not
to be confused with vk::PipelineCache) implements a cache of such
pipeline objects.

In preparation for Compute support, these data structures are prefixed
with Graphics.

Bug: angleproject:2959
Change-Id: I9181586fb946b787216ca0b2ad6340f90c3ab55f
Reviewed-on: https://chromium-review.googlesource.com/c/1333971Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>