snapshot: Allow passing the bounds of the created paintable

snapshot: Allow passing the bounds of the created paintable

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.