Upstream Apple's direct-to-Metal backend: compile libANGLE. This change is meant to merge the metal backend changes from Apple's direct-to-Metal backend. Taken from Kyle Piddington's CL: https://chromium-review.googlesource.com/c/angle/angle/+/2857366/ The goal of this CL is to merge the metal backend code in a state that compiles, but not to switch the Metal backend over to using the direct-to-metal backend yet. Bug: angleproject:5505 Bug: angleproject:6127 Change-Id: If6783e06e0086b3a1dd25c6f53caca5cfc96cb86 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2950067 Reviewed-by: Jonah Ryan-Davis <jonahr@google.com> Reviewed-by: Kenneth Russell <kbr@chromium.org> Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>