VK_KHR_depth_stencil_resolve is used by this change to resolve
depth/stencil multisampled-render-to-texture renderbuffers.

This extension is not widely supported yet.  If it's not present, the
depth/stencil resolve operation is silently ignored and the renderbuffer
acts as a normal multisampled one.  This is not correct, but our primary
user (Chrome), and most applications don't care for the resolved
depth/stencil data.  In fact, it's recommended for the depth/stencil
attachment to be invalidated after rendering.

Exposing EXT_multisampled_render_to_texture even in the absence of
depth/stencil resolve allows the majority of the applications to still
take advantage of MSRTT color attachments.

Bug: angleproject:4836
Change-Id: I6ba4187344a0c9330d2c77bdc5e2c6fc5483c299
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2417645
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>