other infill patterns

[aligator]

I already started experimenting with zig-zag lines (make the lines a bit shorter and connect the ends to get a continous line).
But it's not that easy.

There are also several other possibilities to generate infill. Just look at the prusa slicer for examples.

[aligator]

I implemented a first version of the zig-zag lines. I am not fully satisfied with it and it's not very fast, but it works for now. I plan to use this feature also for the initial support generation.

Still some other patterns can be added and I leave this open for now.