This patch 'fixes' the size_allocate callbacks for the vbox and hbox
widgets. They were broken in the way they divided leftover pixels: all
leftover pixels went to the last elegible widget, which could therefor
be (number of elegible widgets minus one) pixels larger than the other
ones, creating stuff as shown in
http://www.nada.kth.se/~johannes/vbox.gif. This patch uses careful
floating point computations and rounding to spread those leftover
pixels out.