Reduce Path clip Mutators to simpler geometry #164666

Currently if Flutter creates a ClipPath layer, that layer will be passed along to Mutators as a path and most mutators will just convert it to a platform path, but it might be a simpler rect, oval, or rrect. Since the rendering backends tend to reduce the clipping to a simpler form, there may be a mismatch between the paths being expressed and the actual rendering performed as was discovered while implementing #164258

The clip_path_layer could perform the simplification so as to make Mutators and canvas->clip operations more compatible and simplify the implementation logic required in the embedders.