Always postConcat() pipeline gradient matrices

In order to unblock https://skia-review.googlesource.com/c/26564 &
https://skia-review.googlesource.com/c/26427, switch all impls to use a
single/atomic postConcat for the gradient matrix.

This is much easier to guard than the changes above.

Change-Id: Ib72dbc7b5a56a96390f4468fd12caf8b6a0e46ef
Reviewed-on: https://skia-review.googlesource.com/26620
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
4 files changed