Metal: Fix matrix vertex attributes with mixed sources Updated VertexArrayMtl::setupDraw to support matrix vertex attributes that use current and array values simultaneously. Simplified the helper function. Fixed: angleproject:8456 Change-Id: I09a26a978cda4b9ac3747325ad571d5ad2fff72d Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5116500 Commit-Queue: Alexey Knyazev <lexa.knyazev@gmail.com> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Quyen Le <lehoangquyen@chromium.org>