glRenderbufferStorageMultisampleEXT has different semantics from
glRenderbufferStorageMultisample.  This change makes this
differentiation in the front-end and passes a flag to the backends.
This extension is currently only supported on Vulkan and D3D backends.
Support for the former will be done in a follow up.  Support for the
latter is not planned.

Bug: angleproject:4836
Change-Id: I75bc3d7990a4b1ce06264280a386d5e467983b7d
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2405396
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>