Metal: Add maximum render target size validation Bug: angleproject:6790 Change-Id: I4a7e7285e040cc68e4fc16bb653f6d1512f541c9 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3331671 Reviewed-by: Kenneth Russell <kbr@chromium.org> Commit-Queue: Gregg Tavares <gman@chromium.org>