Using the same shader that unresolves color, this change allows
depth/stencil to be unresolved as well.

In turn, this allows the depth and stencil loadOp/storeOp of the
implicit multisampled image associated with a
multisampled-render-to-texture renderbuffer to be set to DONT_CARE.

Stencil unresolve depends on VK_EXT_shader_stencil_export.  In the
absence of this extension, the stencil aspect is not unresolved and must
continue to use loadOp=LOAD and storeOp=STORE.  This is not ideal, but
the expected use-case of depth/stencil MSRTT renderbuffers is that they
get invalidated, so that load and store wouldn't happen in practice.

Bug: angleproject:4836
Change-Id: I9939d1e15e10fa8ed285acdd6fe6edb42c59054f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2427049Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>