Fix 'librsvg' deprecation warnings.

* src/rsvg-port.c (rsvg_port_preset_slot): Use `rsvg_handle_render_document`
instead of `rsvg_handle_render_cairo`, and `rsvg_handle_render_layer`
instead of `rsvg_handle_render_cairo_sub`, as suggested by the warning,
conditionally on newer librsvg 2.52+.

Signed-off-by: Hin-Tak Leung <htl10@users.sourceforge.net>