This change allows the use of resolve attachments in the Vulkan backend.
GL_EXT_multisampled_render_to_texture is implemented using this feature.
The infrastructure for specifying resolve attachments is designed with
eventual support for GL_EXT_multisampled_render_to_texture2 in mind as
well as optimizations to glBlitFramebuffer() and multisampled
backbuffers.

Proper support for glRenderbufferStorageMultisampledEXT is still missing
from this change.  All tests use this for the depth/stencil attachment
and don't read back the data.  Currently, the depth/stencil attachment
is created as a normal multisampled image.

Bug: angleproject:4836
Change-Id: I110a7f63312ae61a657b6094adf7d97c92bd5843
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2304170
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>