gst/alpha/gstalpha.c: Implement alpha functions for AYUV too, this increases accuracy...

gst/alpha/gstalpha.c: Implement alpha functions for AYUV too, this increases accuracy quite a bit.

Original commit message from CVS:
* gst/alpha/gstalpha.c: (gst_alpha_method_get_type),
(gst_alpha_set_property), (gst_alpha_sink_link),
(gst_alpha_set_ayuv), (gst_alpha_set_i420),
(gst_alpha_chroma_key_ayuv), (gst_alpha_chroma_key_i420),
(gst_alpha_init_params), (gst_alpha_chain):
Implement alpha functions for AYUV too, this increases
accuracy quite a bit.