* rsvg.c: Use art_render_mask in libart.

	* Makefile.am:
	* art_render_mask.c: Remove.
	* art_render_mask.h: Remove.
	Get rid of our own copy of art_render_mask.

	* configure.in: Change back so compatible with older autoconf
	and automake. We'll go back to the future when glib does.