GL_R16UI format is not supported yet. It will be implemented once
integer textures are implemented in metal back-end.

Bug: angleproject:4847
Bug: angleproject:2634
Change-Id: I60a52c0ce327a524c74e80b18bb15978ac52065b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2329091
Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>