Metal: Enable OES_fbo_render_mipmap Bug: angleproject:6813 Change-Id: I643c114d7b37ca0532d2dda1964090a15e470842 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3342568 Reviewed-by: Kyle Piddington <kpiddington@apple.com> Reviewed-by: Kenneth Russell <kbr@chromium.org> Commit-Queue: Alexey Knyazev <lexa.knyazev@gmail.com>