[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/tools/viewer/SkottieSlide.cpp b/tools/viewer/SkottieSlide.cpp
index aef28e1..77501d8 100644
--- a/tools/viewer/SkottieSlide.cpp
+++ b/tools/viewer/SkottieSlide.cpp
@@ -17,9 +17,9 @@
 }
 
 void SkottieSlide::load(SkScalar w, SkScalar h) {
-    fAnimation  = skottie::Animation::MakeFromFile(fPath.c_str());
-    fWinSize    = SkSize::Make(w, h);
-    fTimeBase   = 0; // force a time reset
+    fAnimation = skottie::Animation::MakeFromFile(fPath.c_str());
+    fWinSize   = SkSize::Make(w, h);
+    fTimeBase  = 0; // force a time reset
 
     if (fAnimation) {
         fAnimation->setShowInval(fShowAnimationInval);