Rethink positive/negative layering

[SjoerdOptLand]

Currently, there is an infinite list of even (positive) and odd (negative) layers in an Image. To avoid collisions we now adopt a layer map:

0-9: ground plane with gaps and the like
10-19: traces
20-29: vias

The inconvenience being the inflexibility of the number of layers. Too many, because some parsers don't like empty layers. Too little, because we might want to draw complicated stuff...

How about adding layers for every object (in draw() order) and finally merging equal layers and removing empty layers?