[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>
diff --git a/dm/DMSrcSink.h b/dm/DMSrcSink.h
index 157a484..1ad2eca 100644
--- a/dm/DMSrcSink.h
+++ b/dm/DMSrcSink.h
@@ -274,11 +274,11 @@
 
 private:
     // Generates a kTileCount x kTileCount filmstrip with evenly distributed frames.
-    static constexpr int               kTileCount = 5;
+    static constexpr int      kTileCount = 5;
 
-    Name                               fName;
-    SkISize                            fTileSize = SkISize::MakeEmpty();
-    std::unique_ptr<skottie::Animation> fAnimation;
+    Name                      fName;
+    SkISize                   fTileSize = SkISize::MakeEmpty();
+    sk_sp<skottie::Animation> fAnimation;
 };
 #endif