BackendInterface is 2D only

[mozman]

@mbway FYI The BackendInterface class and all derived backends have been modified to support only 2D shapes.

All current backends are 2D only and this change allows further optimizations by switching to numpy based 2D shapes.

[mozman]

I extracted the Designer class from the frontend module, converted Designer to an abstract base class and added a new Designer2d class. The conversion from 3D to 2D is done in Designer and not in the Frontend, so it's possible to use the Frontend for 3D rendering in the future by creating a Designer3d class.

[mozman]

I replaced the Path2d class by NumpyPath2d and removed Path2d for simplicity. I optimized the throughput of the Designer/Backend stages and noticed that none of the existing backends require the separation of filled paths into exterior paths and holes and counter-clockwise and clockwise orientation. Removing this pre-processing step sped up text rendering a lot and maybe other tasks as well.

I will simplify the BackendInterface.draw_filled_paths() method to draw_filled_paths(paths) and add a helper function oriented_paths() which can separate and orient path if a backend requires such a preparation.