Code review comment for lp:~gaspa/a4/presentation-objects

List of bugs:
1- all rectangles are always drawn from the presentation upper edge.
2- all rectangles have a lower border way bigger than other ones
3- very small rectangles are badly drawn
4- rectangles have weird starting points (I mean: there is a weird correlation between the point you begin to draw and the place the rectangle is actually drawn)
5- when a selection is drawn downside up, the rectangle is drawn outside of the presentation, starting from the upper edge. Further, the black part of the rectangle is bigger than the white part