DrawingCtx.compute_path_extents(): new function
DrawingCtx.compute_path_extents(): new function

Let's try computing a shape's extents, to form its bounding box,
outside of the drawing code.  First, a function to call
cairo_path_extents().

In the future we can do this with the kurbo crate, hopefully.

Part-of: <https://gitlab.gnome.org/GNOME/librsvg/-/merge_requests/757>

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