Fix tiling gap size when exporting

During export, a new LPETiling() instance was being created, with a default "prev_unit" value of "px".

This prev_unit value was then compared to the current unit value, and the gap size was converted to the new unit. But on instantiation, the unit and gap size were already correct. So this applied a pointless extra conversion from px to the current unit.

Fixes #6157 (closed)