modernDimmerPosition indexed dimPositions without a range check in
RaLightsSet(), SetLightDimmings() and the off_from_dimmer value of the
cab's Python state. An empty DimmerList, or a DefaultPos past the end of
a short one, read past the end of the vector; the dimmer commands could
also leave or put the position outside the list.
Reads go through currentDimPosition(), a plain entry (not off, not
dimmed) outside the list. DefaultPos and the position are clamped after
the .fiz is loaded, the increase/decrease commands step through
stepDimmerPosition(), which clamps and does nothing for an empty list,
and headlightsdimenable clamps its position 1.
