stage: Adjust drawing to include the window scaling factor

In order to transparently support high DPI density displays, we must
maintain all coordinates and sizes exactly as they are now — but draw
them on a surface that is scaled up by a certain factor. In order to
do that we have to change the viewport and initial transformation
matrix so that they are scaled up by the same factor.

https://bugzilla.gnome.org/show_bug.cgi?id=705915