Add MoltenVK support to Viewer.

Works with v1.0.17, will probably need updating for later revisions.

Bug: skia:8737
Change-Id: I9e42fad90656a88efa12625856019a8282ff39fd
Reviewed-on: https://skia-review.googlesource.com/c/191298
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
4 files changed