[sksg] Add 3x3 Concat Transform specialization

When both inputs are 3x3, use 3x3 Concat.

TBR=
Change-Id: Ife6a98792c2ed57a2a987fe5d0bdf265aeb64b0a
Reviewed-on: https://skia-review.googlesource.com/c/182968
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
4 files changed