(#1036): Don't have the main rsvg crate depend on gdk-pixbuf
Inline Surface.to_pixbuf() into librsvg-c/src/pixbuf_utils.rs, its only caller.
This is the only place where to_pixbuf() is called, and doing this lets us remove the gdk-pixbuf dependency from the main rsvg crate. Only librsvg-c has the gdk-pixbuf dependency now.
Fixes #1036 (closed)