Metal: Disable multisampled render to texture. Corruption has been seen on M1 Macs for WebGL content which uses a multisampled backbuffer. Bug: chromium:358957665, angleproject:42261786 Change-Id: Ia17581b15cd032e91e2642d80abace8d57ceb23b Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5783540 Commit-Queue: Geoff Lang <geofflang@chromium.org> Reviewed-by: Quyen Le <lehoangquyen@chromium.org>