[skottie] Support arbitrary displacement map sources

Currently, displacement maps only work with sources preceding the
displacement effect (layers higher up in the stack which are already
attached when the effect is evaluated).

Using the utils introduced in
https://skia-review.googlesource.com/c/skia/+/935556, we can now
support arbitrary source layers.

Change-Id: I10d49c276974832808436c12c53922be6deb38d4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/935543
Reviewed-by: Brandon Wilson <bwils@google.com>
Commit-Queue: Florin Malita <fmalita@google.com>
5 files changed