bugfix flow inaccuracies in FillSmooth.
TODO: do something about surface too small to be printed, particularly when 'only one perimeter top' -> don't extract them or use gapfill
The gap fill was disabled for zero infill.
Now the gap fill is enabled in between the perimeters, but disabled between
the inner-most perimeter and infill in case the infill is set to zero.
Also in case there are multiple infill regions inside a perimeter,
the mutliple infills are considered as non-zero if at least one infill
is non-zero, therefore the gap fill will be added inside the inner-most
perimeter.
