Return computed matrices by value, allowing const and temp values

Change-Id: I178428c0b9f12df36a33a1da2999b5bd42e72ab8
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1023096
Reviewed-by: Florin Malita <fmalita@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>