What this means is basically that a vertically oriented GtkCellAreaBox
will render cells properly even if the height is not constant for every
for of data in the said GtkCellAreaContext (i.e. the height was not allocated
by gtk_cell_area_context_allocate).

This is done completely on the fly and so is much more heavy duty
at render time (considerably slower but not visibly noticable in
lightweight views like GtkTreeMenu). Note that cell alignments
are not possible in an unallocated orientation, each row of data
individually receives only enough space to render the independant
row and no space is reserved for alignments if the size is not
a constant size across rows in the same context.