Support for Multiple Surfaces

[LPGhatguy]

Currently, yakui assumes that UI is drawn in one coordinate space, on a subset of one contiguous rectangular surface.

It's often useful to draw UI in multiple different places, like projected in the world, in a separate window, or in a scrollable and zoomable canvas.

[LPGhatguy]

The PaintLayer API introduced in 92e0b27 might be a good step in this direction. We could add a window or surface ID to a layer and use that to dispatch calls to the right places.