[gta02-core] Solder mask and other special pads

[Werner Almesberger]

I wrote:
> In a future version, I'll make fped
> automatically remove the solder mask hole if there's a solder mask
> "pad" that overlaps with the copper.

The future is now :-)

This also adds some sanity checks, so if you happen to place copper
pads on each other or dab solder paste outside the copper area of a
pad, fped will complain.

At the moment, it will let areas of solder paste pass if they're
completely outside of any copper, so that one doesn't get blue
screens when setting things up step by step.

- Werner