DrawingCtx: Keep the whole stack of viewboxes in the view_box_stack
DrawingCtx: Keep the whole stack of viewboxes in the view_box_stack

We used to keep the element at the top of the stack in a separate
field, which is a bit awkward.

GNOME/librsvg,GNOME/librsvg,GNOME/librsvg,GNOME/librsvg