rsvg-load.c: Make all the SAX callbacks take an RsvgXmlState, not an RsvgLoad
rsvg-load.c: Make all the SAX callbacks take an RsvgXmlState, not an RsvgLoad

The callbacks are all independent of the RsvgLoad; they are only
little trampolines into the Rust code.  Now we can move them all to
Rust in a single shot.

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