Rounded corners on custom pad shapes (lp:#1777516)

Original report created by Evan Shultz (evan-shultz)

Please see https://github.com/KiCad/kicad-footprints/issues/658.

IPC 7351C will be recommending rounded rectangle pads. (Apparently this was
desirable from the first release of the document but ECAD software didn't
commonly support this pad shape so oblong was the recommendation.) While we
can use rounded corners with rectangular KiCad pads, custom pad shapes do
not allow rounded corners.

This prevents all pads from having rounded corners and makes a choice
between not adhering the IPC's latest recommendations or having pads both
with and without rounded corners (if only custom pad shapes aren't
rounded). The example given in the link above is for a QFN (currently
Package_DFN_QFN:QFN-28_4x4mm_P0.5mm) with funky corner pads that are best
represented with custom pad shapes.

The librarians would like to have rounded corners available on custom pads
so that the benefits of rounded corners are available to all pads, and all
pads in KiCad can be compliant with the best practices according the IPC
7351C.