The overlay surfaces are going to be the same IOSSurface implementation
with the platform views controller set to null (so these are surfaces
that don't support embedding platform views to them).

  * Adds a FlutterOverlayView which is a UIView that's showing an
    overlay surface.
  * Creates an overlay surface for each embedded UIView (done in
    FlutterPlatformViewsController).
  * Changes CompositeEmbeddedView to return a new canvas.
  * Makes the PlatformViewLayer replace the PaintContext's canvas with
    the canvas for the overlay view.
  * Changed canvas in PaintContext to be a pointer so it can be changed.

TBD in following PRs:
  * Copy the current canvas state when replacing a canvas in PaintContext.
  * Make FlutterOverlayView work with a GL backend (currently it only
    works with software rendering)