filters/image.rs: Parse the href at loading time, in set_atts()
filters/image.rs: Parse the href at loading time, in set_atts()

Instead of at rendering time.  This makes the rendering code easier.
In the end we'll move it to actually loading external images during
the loading stage.

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