Add Context::renderbufferStorageMultisampleEXT 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/+/2405395 Reviewed-by: Jonah Ryan-Davis <jonahr@google.com> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>