Metal backend implementation pt 2 This is without Metal specific shader translator implemented yet. Bug: angleproject:2634 Change-Id: I95d589442251c9ba111bd05a2dc379a36739046c Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1855069 Reviewed-by: Jonah Ryan-Davis <jonahr@google.com> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>