Add smarter background handling

[neinseg]

Currently, when converting an SVG file with black background and white foreground using --format gerber will produce clear gerber primitives on top of clear background in the output. We should figure out a way to handle this properly in all cases.

Simply pasting a rect of the same color as the background behind everything will not work properly since that may actually change the bounding box of the design. Possibly the best way to deal with this might be to add some logic to catch this case that will advertise the --flip-gerber-polarity flag.

See #13.