rsvg-port.c: Simplify usage of `rsvg_handle_render_*`.

* src/rsvg-port.c (rsvg_port_preset_slot): Remove usage of
`rsvg_handle_render_document` (and `rsvg_handle_render_cairo`).  They are
functionally the same as `rsvg_handle_render_layer` (and
`rsvg_handle_render_cairo_sub`) with argument id set to NULL.

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