MSL doesn't support array of matrices as varying variable.
This CL converts array of matrices varying to array of vectors.

This fixes WebGL test:
conformance/glsl/misc/shader-varying-packing-restrictions.html

Bug: angleproject:4846
Bug: angleproject:2634
Change-Id: I3cfc638dc01092ddf1add7df02c131d1a5c8ba1d
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2392172
Commit-Queue: Le Hoang Quyen <le.hoang.q@gmail.com>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>