For percent-based configuration in the form of
middle button 33%-66%, right button 66%-0 we'd get an error because of the
one device unit overlap. This was neither documented nor useful, because
leaving a 1% gap leaves an actual gap between the buttons. Allow for an
overlap of one device unit on the edge of the buttons.

What's picked in that case depends on the implementation of
is_inside_softbutton_area but since one device unit is so small, it doesn't
matter.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>