* Fixed ChildSceneLayer elevation issue on Fuchsia.
ChildSceneLayers would draw on top of stacked Flutter content on
Fuchsia if not wrapped up in a Material widget, i.e. a
PhysicalShapeLayer. This patch pushes the logic from there to all
types of Layers.
Flow is a simple compositor based on Skia that the Flutter engine uses to cache
recoded paint commands and pixels generated from those recordings. Flow runs on
the raster thread and uploads information to Skia.