[sksg] Hit-testing API

Introduce RenderNode::nodeAt(const SkPoint&) as the entry point for the hit-testing API.

This is backed by a onNodeAt() virtual, which gets dispatched throughout the render DAG,
and normally stops at the first leaf Draw node in encounters.

To support the implementation, introduce a GeometryNode::contains(const SkPoint&) API.

This is backed by a onContains() virtual, overridden in each concrete geometry class.

Expose nodeAt() on sksg::Scene, and add some basic unit tests.

Change-Id: I0c8abd9d1e51ecf2d8b4dd699f325cd636e21084
Reviewed-on: https://skia-review.googlesource.com/c/191296
Commit-Queue: Florin Malita <fmalita@chromium.org>
Reviewed-by: Mike Reed <reed@google.com>
diff --git a/modules/sksg/include/SkSGClipEffect.h b/modules/sksg/include/SkSGClipEffect.h
index 52ba7c7..309127f 100644
--- a/modules/sksg/include/SkSGClipEffect.h
+++ b/modules/sksg/include/SkSGClipEffect.h
@@ -33,6 +33,7 @@
     ClipEffect(sk_sp<RenderNode>, sk_sp<GeometryNode>, bool aa);
 
     void onRender(SkCanvas*, const RenderContext*) const override;
+    const RenderNode* onNodeAt(const SkPoint&)     const override;
 
     SkRect onRevalidate(InvalidationController*, const SkMatrix&) override;