When using maxlevel, column view exporting is aborted when the first
headline below maxlevel is reached.  This prevents any remaining
headlines that are at or above maxlevel from being rendered.

The documentation suggests that it is actually used to export a column
view for all headlines that are at or above the value of maxlevel.

This patch adjusts the loop condition so that it no longer checks
maxlevel, and moves the maxlevel condition inside the loop.  With this
change, all headlines at or above maxlevel are correctly exported.