Initial scene graph (SkSG)

Sketching a thin (as in close-to-skia-semantics) scene graph API, focused on
external animation, inval tracking and minimal repaint.

Only a few concrete classes/features so far:

* Rect/Color/Transform/Group
* basic inval tracking
* a trivial animated sample with inval visualization

Pretty much everything (especially naming) is volatile, so treat accordingly.

The interesting bits to review are likely in Node.{h,cpp} for inval and
SampleSGInval.cpp for usage.

Initial class hierarchy:

  * Node: invalidation/ancestors tracking
  |
   -- * RenderNode: onRender(SkCanvas)
  |   |
  |    -- * Draw (concrete): rendering a [geometry, paint] tuple
  |   |
  |    -- * Group (concrete): grouping multiple RenderNodes
  |   |
  |    -- * EffectNode: single-descendant effect wrapper
  |       |
  |        -- * Transform (concrete): transform effect
  |
   -- * PaintNode: onMakePaint()
  |   |
  |    -- * Color (concrete): SkColor paint wrapper
  |
   -- * GeometryNode: onComputeBounds(), onDraw(SkCanvas, SkPaint)
      |
       -- * Rect (concrete): SkRect wrapper

TBR=

Change-Id: Iacf9b773c181a7582ecd31ee968562f179d1aa1b
Reviewed-on: https://skia-review.googlesource.com/85502
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
diff --git a/experimental/sksg/SkSGGeometryNode.h b/experimental/sksg/SkSGGeometryNode.h
new file mode 100644
index 0000000..52b2df0
--- /dev/null
+++ b/experimental/sksg/SkSGGeometryNode.h
@@ -0,0 +1,51 @@
+/*
+ * Copyright 2017 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef SkSGGeometryNode_DEFINED
+#define SkSGGeometryNode_DEFINED
+
+#include "SkSGNode.h"
+
+#include "SkRect.h"
+
+class SkCanvas;
+class SkPaint;
+
+namespace sksg {
+
+/**
+ * Base class for nodes which provide 'geometry' (as opposed to paint)
+ * for drawing.
+ *
+ * Think SkRect, SkPath, etc.
+ */
+class GeometryNode : public Node {
+public:
+    void draw(SkCanvas*, const SkPaint&) const;
+
+    // SkPath asPath() const;  // unused for now
+
+protected:
+    GeometryNode();
+
+    virtual void onDraw(SkCanvas*, const SkPaint&) const = 0;
+
+    virtual SkRect onComputeBounds() const = 0;
+
+    // virtual SkPath onAsPath() const = 0; // unused for now
+
+    void onRevalidate(InvalidationController*, const SkMatrix&) override;
+
+private:
+    SkRect fBounds;
+
+    typedef Node INHERITED;
+};
+
+} // namespace sksg
+
+#endif // SkSGGeometryNode_DEFINED