This simplifies the pipeline state object caching. We will not need
to use any extra bits to cache based on program properties - instead
all programs will be compatible. The pipeline layout strucutre is
described in the design docs. It currently only has two bind groups:
the first for default uniforms, and the second for Textures. In the
future we might re-organize this to handle driver uniforms, dynamic
push constants, and/or program uniform buffers with ES 3.0.

Instead of storing only the Textures that are required by a Program,
we reserve space for the maximum possible Texture units. We might have
to revisit this very simple design in the future to support texture
arrays, which are handled specially in Vulkan.

Bug: angleproject:2163
Change-Id: I3e1656c2c73045aed56838a5f1267b246a623362
Reviewed-on: https://chromium-review.googlesource.com/837943Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>