Swapping out renderers

[corranwebster]

Currently all rendering is being done with Micropython's framebuf module, which provides basic drawing, but lacks some features such as multipolygons, winding number fill, etc. There are other libraries out there which may provide better support, but most of them require C extensions.

However, if they are available, it might be nice to be able to support rendering with those instead of framebuf. Probably the way to do this is to make the Raster object richer and act as a drawing intermediary - Shapes call to the Raster for actual drawing of individual geometries, and they then render as best they can.

This would align well with supporting alternate pixel formats, such as opacity, 8-bit grayscale, etc. via the Raster object.