This is a little bit like the bounding hull tool, but the
output is "exact" and it only supports the most common
source items.
By 'exact', this means that rounded corners are real arc
segments rather than polygonal approximations. Obviously,
this is rather tricky in the general case, and especially
for any concave shape or anything with a bezier in it.
Envisioned main uses:
* Creating courtyard and silkscreen offsets in footprints
* Making slots around line or arcs.
The one thing that it does not currently do, but which it might
plausibly do without reimplementing Clipper is convex polygons,
which would bring trapezoidal pad outsets for free. But that
is a stretch goal, and bounding hull can be used.
