[skottie] Nested animation support

Extend composition layers to support referencing external .json
animations ("$"<PATH> syntax).

This is a custom extension (not supported in BM/Lottie).

Also make skottie::Animation ref-counted, to facilitate sharing.

TBR=

Change-Id: I062d031e5868d759f3930dea9b261f9b3ec81684
Reviewed-on: https://skia-review.googlesource.com/109806
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
1 file changed