Vulkan: Implement basic depth/stencil buffers. 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>