[skrive] Initial artboard plumbing

"Artboards" are top-level Rive containers (similar to AE compositions),
holding the scene graphics and related animations.

Artboard properties:

  - name
  - width/height (size)
  - translation (position)
  - origin (anchor point for transforms?)
  - (background) color
  - clip contents flag

Plumb artboard parsing + background rendering, and hook into viewer.

TBR=
Change-Id: Ib188245ce41a76197cf9e0937689adf8243826d6
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/295244
Reviewed-by: Florin Malita <fmalita@google.com>
Commit-Queue: Florin Malita <fmalita@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index 381969f..fd207c4 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -2300,6 +2300,7 @@
         "tools/viewer/ParticlesSlide.cpp",
         "tools/viewer/SKPSlide.cpp",
         "tools/viewer/SampleSlide.cpp",
+        "tools/viewer/SkRiveSlide.cpp",
         "tools/viewer/SkSLSlide.cpp",
         "tools/viewer/SkottieSlide.cpp",
         "tools/viewer/SlideDir.cpp",
@@ -2322,6 +2323,7 @@
         ":skia",
         ":tool_utils",
         ":trace",
+        "experimental/skrive",
         "modules/particles",
         "modules/skottie",
         "modules/skottie:utils",