Considering the operations that some of the rendernode constructors
do, nodes with width or height 0 (or both of course) are very well
possible. This would break in the rendernodepaintable when adding a
transform, which divides by width/height of the rendernode.
This allows being more specific about the size.
It's useful in particular when the resulting render nodes might be
too small for the size, not only when they are too large. For the
latter case, using a clip node would be enough.
It also requires adding a clip node when rendering the resulting
paintable, but that should be optimized out by GtkSnapshot when not
necessary.
