Add support for external precomp Skottie layers. This allows embedders
to seamlessly mix custom/Lottie content.
General flow:
* embedders register a PrecompInterceptor callback with
the animation builder
* at build time, Skottie invokes the callback for each pre-composed
layer
- the returned ExternalLayer implementation is used instead of the
Lottie layer payload
- (a nullptr value signals Skottie to use the usual Lottie payload)
* at render time, ExternalLayer::render() is called to defer content
rendering to the embedder
Also implement a sample PrecompInterceptor which attempts to substitute
precmp layers matching a given pattern with external Lottie animations:
precomp_name: "__foo.json" -> Animation("foo.json")
This new mechanism is a generalization of (and supersedes) the old
NestedAnimation hack - so we can remove that.
Change-Id: Id80fe11881c62b8717c2476117c7c03ad5300eef
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/288130
Commit-Queue: Florin Malita <fmalita@chromium.org>
Reviewed-by: Mike Reed <reed@google.com>
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