* [fuchsia] Add labels to Scenic nodes.

* [fuchsia] Skip creating Scenic nodes for identity Transforms.

* [fuchsia] Assign elevation to Scenic nodes based on paint order.

* [fuchsia] Create Scenic OpacityNodes at leaf nodes.

* [fuchsia] Composite PhysicalShapeLayers using Skia, except when they need to float above child views.

In that case, they will still need to be pulled
into separate Scenic nodes to be composited on top
of the child view[s].

* [fuchsia] Add tests for Fuchsia-specific layer behavior.

Inspect commands going to Scenic and make sure
they match what is expected.

Also, restructure code to need less member variables,
and other cleanups based on review feedback.