Add VMnc decoder.

Add VMnc decoder.

Original commit message from CVS:
* configure.ac:
* gst/vmnc/Makefile.am:
* gst/vmnc/vmncdec.c: (gst_vmnc_dec_base_init),
(gst_vmnc_dec_class_init), (gst_vmnc_dec_init),
(gst_vmnc_dec_reset), (vmnc_handle_wmvi_rectangle),
(render_colour_cursor), (render_cursor), (vmnc_make_buffer),
(vmnc_handle_wmvd_rectangle), (vmnc_handle_wmve_rectangle),
(vmnc_handle_wmvf_rectangle), (vmnc_handle_wmvg_rectangle),
(vmnc_handle_wmvh_rectangle), (vmnc_handle_wmvj_rectangle),
(render_raw_tile), (render_subrect), (vmnc_handle_raw_rectangle),
(vmnc_handle_hextile_rectangle), (vmnc_handle_packet),
(vmnc_dec_setcaps), (vmnc_dec_chain), (vmnc_dec_change_state),
(vmnc_dec_set_property), (vmnc_dec_get_property), (plugin_init):
Add VMnc decoder.
Still missing support for:
- rectangle types I didn't find in my samples (e.g. copy, RRE,
ZRLE)
- alpha-composited cursors