Metal: Support array of matrices varying. 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>