[librsvg: 1/5] Start branch explicit-bboxes

[Federico Mena Quintero <federico src gnome org>]

commit eb0c872b08201678244a0b63a976894abe6015dd
Author: Federico Mena Quintero <federico gnome org>
Date:   Tue Sep 17 20:40:36 2019 -0500

    Start branch explicit-bboxes
    
    Currently the bboxes get computed at the innermost stroke_and_fill(),
    for example, and inserted immediately into the DrawingCtx's via
    dc.insert_bbox().
    
    Also, the drawing functions and helpers like stroke_and_fill()
    currently return a Result<(), RenderingError>.
    
    To remove some mutability from DrawingCtx, this branch will try to
    make those functions return Result<BoundingBox, RenderingError>
    instead.  The bbox will get accumulated up to the toplevel code.