This implements basic depth/stencil states and clearing.

This also implements "fallback" texture formats in the texture
generation. Fallback formats are those that are chosen at runtime for
replacements for main formats which lack driver support. They are
different from override formats, which are always used because we
assume there is no driver support.

The Vulkan spec only asserts that one of the two of D32 or D24 has
mandatory support. In the case of AMD, D24 is not supported fully, and
we need the fallback format support emulation.

Bug: angleproject:2357
Change-Id: Ic86cede3c69ff9893a06b3a55c3b5d2d897fa55f
Reviewed-on: https://chromium-review.googlesource.com/916701
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Frank Henigman <fjhenigman@chromium.org>