glRenderbufferStorageMultisampleEXT is special in that it doesn't have
the same semantics as glRenderbufferStorageMultisample, but is aliased
in gl.xml for the sake of desktop GL (where it does have the same
semantics).  This change special-cases this function so
multisampled-render-to-texture renderbuffers can be supported.

Bug: angleproject:4881
Change-Id: Iaa321dd0671dd82ea802df22a1ae935942789c21
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2405395Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>