Add `rc` accessor method to SkMatrix.

FP gencode reads back matrix values in its dumpInfo method using rc().
This worked for SkM44; now it will work for SkMatrix as well.

Change-Id: I4efc7018529bd3c84aacc073fad2bfca7e12c517
Bug: skia:10748
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/318756
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
3 files changed