This PR adds hit testing to Flutter by overriding `hitTestSelf` on the Rive RenderObject.
Currently, the hit area is the entire bounding box of the canvas. This means that when a Rive animation is rendered above any other Flutter content (for example, a Stack) all hits are absorbed by Rive and do not pass through.
With this change, Rive will only absorb hits if the pointer comes in contact with a hittable Rive element.
With this change, `handleEvent` will only be called if `hitTestSelf` returns true. There is some duplicate work here as `_processEvent` already performs similar hit test logic, which we can look at optimizing. But `hitTest` needed to be separate method call, as `hitTestSelf` is called before `handleEvent` and `handleEvent` sends additional information (whether it's a pointer down/up etc.).
Diffs=
95beaa4f5 feat: add flutter hit test self on rive render object (#6341)
bd71143bc chore: fix broken docs link (#6360)
Co-authored-by: Gordon <pggordonhayes@gmail.com>
