Replace the explicit bbox_stack by an implicit one
Replace the explicit bbox_stack by an implicit one

A bbox is just pushed in the case where we need a temporary surface
for rendering (i.e. if there are filters/masks/etc.).  This new bbox
has the affine relative to the temporary surface.

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