[skottie] Caching motion tile

Update MotionTileEffect to avoid rebuilding shaders redundantly,
at render time:

  1) build all shaders at revalidation time
  2) cache the layer content picture separately, and only rebuild when
     the layer content changes

To support #2, add some SG helpers for querying subtree inval state.

With this change, we avoid all render time allocations.

Notry: true
Change-Id: I55a1f95752704af6a667b266e725492de6640387
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/226512
Commit-Queue: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Reed <reed@google.com>
Auto-Submit: Florin Malita <fmalita@chromium.org>
Reviewed-by: Mike Reed <reed@google.com>
7 files changed