Use `writeComposite` to build uniform matrices.

`writeComposite` can write and deduplicate constants, so it's preferable
to manually emitting an OpCompositeConstruct opcode.

Change-Id: I0c4ac8f8a456c8561c0b6a90cd316934f20895e8
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/528638
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
13 files changed