Currently this won't speed up performance much, if at all, since we
don't even really support state changes. It sets the stage for using
a pipeline cache later, with better state change support. It also
makes implementing descriptor sets for Textures a bit simpler, since
we can just update descriptor sets when the dirty bits tell us of a
Texture change.

Add cache structures to VertexArrayVk and ContextVk so we only need
to update the structures before we create a new pipeline. When we
support pipeline caching, we will most likely be updating a compact
representation for fast cache query.

BUG=angleproject:1898
BUG=angleproject:2167

Change-Id: Id545f2c67c06d8b6e8b7eb63ca70464f6b9a51f6
Reviewed-on: https://chromium-review.googlesource.com/713586
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Frank Henigman <fjhenigman@chromium.org>