Previously spirv-cross had problems translate mat2 to equivalent
Metal type. See the reported issue:
https://github.com/KhronosGroup/SPIRV-Cross/issues/1200

It has been fixed on upstream repo.
DEQP tests using mat2 should pass now.

Bug: angleproject:2634
Change-Id: I68e871965c7004ae34dc0fa92c926905cb0c14cf
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1966183
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>