[skottie] Add a tool for dumping encoded animation frames

Exports animation frames as a sequence of .png files:

  0000000.png
  0000001.png
  ...

Usage:

  skottie_tool -i <input_json> -w <out_dir>

Other options:

  --width  Frame width (default: 800)
  --height Frame height (default: 600)
  --fps    Frames per second (default: 30)
  --t0     Normalized timeline start (default: 0)
  --t1     Normalized timeline end (default: 1)

Change-Id: I4a79be0f823da15e6863909b6d67d38aa74bb740
Reviewed-on: https://skia-review.googlesource.com/132265
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
diff --git a/BUILD.gn b/BUILD.gn
index 878ee46..4c53f0c 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -1514,6 +1514,14 @@
     ]
   }
 
+  if (is_linux || is_mac || is_win) {
+    test_app("skottie_tool") {
+      deps = [
+        "modules/skottie:tool",
+      ]
+    }
+  }
+
   if (target_cpu != "wasm") {
     import("gn/samples.gni")
     test_lib("samples") {