This change lets us create egl::Surfaces from a D24S8 config. This is
a bit hacky, because the spec only mandates 24 -or- 32 bit depth
support, but not both or either individually. Will need follow-up work
for proper EGL config setup.

A single depth buffer is allocated for the entire set of swapchain
images and is used with each. This also might be a problem if we're
rendering to multiple frames at the same time. We'll likely have to
revisit this in the future as well.

This adds a new RenderTargetVk to the SurfaceVk class which points to
the Depth/Stencil image. Since ImageViews must refer to either the
depth or stencil, but not both, we'll need to address this when we
get to implementing depth/stencil texture reads in shaders.

Bug: angleproject:2357
Change-Id: Ibed0eed7e1d0efb272758dbfc79fa2c5aa93997f
Reviewed-on: https://chromium-review.googlesource.com/919761Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>