This implements masked color clear using clear shaders. The shaders
themselves were introduced in a prior patch. In order to get the
right setup for the draw call to trigger the shaders, we create
an internal pipeline from the pipeline cache. We also use a special
pipeline layout with only uniform buffers. The masked out color
channels are disabled via settings on the pipeline.

This fixes the dEQP masked color clear tests. It doesn't handle
masked color clears combined with the depth clear bit. It's likely
we don't have test coverage for this case.

Bug: angleproject:2455
Change-Id: I513248cc0f7e58f490fc16ac9afb40119d730ccc
Reviewed-on: https://chromium-review.googlesource.com/1031373
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>