Fix compilation of image.c on MS-Windows

Fix compilation of image.c on MS-Windows

This is a followup to last change in image.c.
* src/image.c (rsvg_handle_get_intrinsic_dimensions): Define to
call fn_rsvg_handle_get_intrinsic_dimensions.
(svg_css_length_to_pixels): Compile only for librsvg >= 2.46.0, as
RsvgLength type was not defined before.