When we visit a PlatformViewLayer during the paint traversal it replaces
the PaintContext's canvas with a new one that is painted ontop of the
embedded view.
We need to make sure that operations applied by parent layers are also
applied to the new canvas.

To achieve this we collect all the canvases in a SkNWayCanvas and use
this canvas by non leaf nodes. Leaf nodes still paint only to the "current"
canvas.

This PR moves the overlay canvas creation from the paint phase to the
preroll phase, collects them into a SkNWayCanvas and set it in
PaintContext.

To keep this PR focused, I only used the internal_nodes_canvas in the
tranform_layer.
Will followup with a PR that changes all internal layers to use the
internal_nodes_canvas.