[skottie] Cache attached assets

This avoids redundant instantiations for assets referenced multiple
times.

TBR=

Change-Id: I8f61f73e695f0d567e55ef077c7d3fb344399f12
Reviewed-on: https://skia-review.googlesource.com/101002
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
1 file changed