The previous change that implemented EXT_multisampled_render_to_texture
already provisioned this extension in the Vulkan backend.  This change
implements the front-end for this extension and enables it in the Vulkan
backend.

Bug: angleproject:4836
Change-Id: I7080260972e61727c5716051c236f635668cb67b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2330510
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>