Previously the snap points computed for oval pads didn't get all the
points correct. This breaks out the "find snap points for ovals"
into a function, reworks the logic, adds some tests.
Also adds "extremum points" when the oval isn't exactly H/V.
(cherry picked from commit 78c8de9b08)
Fixes: https://gitlab.com/kicad/code/kicad/-/issues/15594