[gta02-core] Solder mask and other special pads

[Werner Almesberger]

While making beautiful new footprints (*), Rene remarked last week
that we had a few pads that fped couldn't handle well yet.

(*) E.g., WM8753FL. Check them out at
    http://people.openmoko.org/werner/gta02-core/gta02-core-modules.pdf

The easier of the issues was QFN center pads where there should be
one large copper area, on which smaller islets of solder paste are
placed.  An example for this is the audio codec (WM8753FL). Fped was
already capable of handling such a constellation, but it didn't show
it in the GUI, which made it hard to check one's work. It does now,
in orange.

With the latest version of fped, pad types can also be changed
directly from the GUI.


Some complications can be brought by special solder mask patterns.
The way solder masks usually work in KiCad is that a pad in a
module definition specifies that the pad's shape is defined in the
copper (component) and in the solder mask layer. So the solder mask
hole defined by the module has exactly the same size and shape as
the pad.

KiCad then adds a board-specific solder mask clearance to all these
solder mask holes, which yields the real solder mask that goes into
the Gerbers.

This approach works well if we have a PCB-specific standard distance
between solder mask and copper. Things get messy if the footprint
specifies a solder mask clearance that differs wildly from KiCad's
default or that doesn't follow the pad shape.

Rene has added a solder mask pad type to fped, which can be used in
combination with a regular or bare pad to tweak the solder mask.
Fped still adds the solder mask hole for the copper, so only some
shapes are possible. 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.

- Werner