Metal: Add Shadow Sampler Grad workaround for AMD This change allows the direct translator to use emulated gradient shadow compare functions for AMD devices, and devices that don't support shadow compare sampler. Bug: angleproject:5107 Change-Id: I8cc8dcb6e6982531c2a858782869283e6dc97052 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3232818 Commit-Queue: Kyle Piddington <kpiddington@apple.com> Reviewed-by: Jonah Ryan-Davis <jonahr@google.com> Reviewed-by: Kenneth Russell <kbr@chromium.org>