Metal: Fix raster order group AMD workaround The AMD-specific raster order group workaround relied on disabled framebuffer attachments being implemented via all-disabled color masks. Metal backend no longer includes disabled framebuffer attachments in render pass descriptors. Updated the workaround to keep the placeholder attachment enabled and to set its color mask explicitly. Fixed: angleproject:8263 Change-Id: I815ceede84e6ea89eefc10c445768d9d569d0d82 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5208110 Commit-Queue: Alexey Knyazev <lexa.knyazev@gmail.com> Reviewed-by: Kenneth Russell <kbr@chromium.org>