[sksg] Animator, Scene

Relocate some reusable logic from Skottie

TBR=
Change-Id: I8764e666c9f1127ed895ee1d16cd66d052469ac5
Reviewed-on: https://skia-review.googlesource.com/98160
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
diff --git a/BUILD.gn b/BUILD.gn
index 4ad27cb..f6ae98a 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -1385,6 +1385,7 @@
       "experimental/sksg/SkSGNode.cpp",
       "experimental/sksg/SkSGPaintNode.cpp",
       "experimental/sksg/SkSGRenderNode.cpp",
+      "experimental/sksg/SkSGScene.cpp",
       "experimental/sksg/effects/SkSGMaskEffect.cpp",
       "experimental/sksg/effects/SkSGOpacityEffect.cpp",
       "experimental/sksg/effects/SkSGTransform.cpp",