gst/alpha/: Stupid plugin to to RGBA to AYUV conversion because none of the colorspace plugins can handle that yet.

Original commit message from CVS:
* gst/alpha/Makefile.am:
* gst/alpha/gstalphacolor.c: (gst_alpha_color_get_type),
(gst_alpha_color_base_init), (gst_alpha_color_class_init),
(gst_alpha_color_init), (gst_alpha_color_set_property),
(gst_alpha_color_get_property), (gst_alpha_color_sink_link),
(transform), (gst_alpha_color_chain),
(gst_alpha_color_change_state), (plugin_init):
Stupid plugin to to RGBA to AYUV conversion because none of
the colorspace plugins can handle that yet.