Pcbdraw

Changelog

- change the logic of search paths to allow for libraries packages
- add several built-in styles
- fix bug with opaque holes
- add a switch to suppress warning about missing backside of the components

- we migrated from Cairo to wand for exploring PNG images. It should yield higher quality and less troublesome dependencies
- a bug with wrongly oriented pads was fixed
- PcbDraw now subtracts solder mask from silkscreen - just like every manufacturer does
- PcbDraw now correctly draws areas without a solder mask. There is a new parameter for styles, `clad`, which specifies the color of these areas