Apparently hittable is not politically correct.

Also, change the hit testing geometry so that we get our hits at the root.

We push a bit up the tree during preroll to learn whether there system
composited layers below each layer. During update scene, we squash down
to paint tasks, which we execute after publishing the new scene.

Example: $ mojo:launcher mojo:spinning_square

This patch removes almost all //base dependency of //flow. The only dependency
left is on tracing.

Instead, make Layer::Paint take a new PaintContext that has just exactly
the state that it needs, mirroring PrerollContext. Also, rename
PaintContext to CompositorContext because it holds the context for the
whole compositor.

We weren't correctly dividing by the device pixel ratio when asking Mozart to
present our child scene.

Previously we just dropped child scenes on the floor. Now we upload them
to Mozart. However, we just draw them on top of all the other content
and don't apply any clips or blends.

This patch contains Dart bindings for adding a child scene when running in
Mozart. The child scene is currently ignored, but a later patch will actually
incoporate it into the tree.