Add a widget derived from GtkDrawingArea that handles the drawing. It
automaticaly requests rerendering of the page if necessary (i.e on a scroll and
rotate).

This should solve all the rendering issues.

Thanks to egl209 for the patch.

Modes are no longer combined to avoid conflicts while
trying to remap one of them

This function should be used to recalculate the coordinates of
a rectangle to its new coordinates on the rendered page
(depending on the current scale and rotation value)