Decorations system

[rafern]

original notes:

• Shadows are painted on the background so the overlay system is not the same as the decorations system
• Maybe extend basecontainer so that it supports decorations, including corner radius etc..., which would optionally use a canvas when needed, or maybe a global clip, although that would probably interfere with widgets that use evenodd clipping (widget.clearstart, widget.clearend)

new notes:

• shadows will actually be kinda complicated to implement without completely wrecking performance
• shadows will probably need to use the layering system nope, we need a system for widgets that paint outside their bounds (maybe paintRect, which defaults to [x, y, width, height]?), and need to capture the child widget in a scratch canvas
• rounded corners are easy now that we have a damage system; just clip when painting

TODO:

• rounded corners
• allow widgets to have painting positions/dimensions different than their layout positions/dimensions
• shadows
• transformations