Ensure that fixed positions always start at 0,0

Ensure that fixed positions always start at 0,0

Fixed positions are defined to be initialized at 0,0 whenever
enabled, by setting fixed_position_enabled to true, or by setting
just one of x/y. This normally happens in the defaults, but we need
to make sure it also happens if a fixed position was once set but
then disabled. We do this by always resetting it back to 0,0 when
fixed_position_set is unset.