configure.ac: Adding videomixer.

configure.ac: Adding videomixer.

Original commit message from CVS:
2005-10-17  Julien MOUTTE  <julien@moutte.net>

* configure.ac: Adding videomixer.
* ext/libpng/gstpngdec.c: (gst_pngdec_class_init),
(user_read_data), (gst_pngdec_chain): More debugging.
* gst/alpha/Makefile.am: Adding alphacolor
* gst/alpha/gstalphacolor.c: (gst_alpha_color_base_init),
(gst_alpha_color_class_init), (gst_alpha_color_init),
(gst_alpha_color_transform_caps), (gst_alpha_color_set_caps),
(transform_rgb), (transform_bgr),
(gst_alpha_color_transform_ip),
(plugin_init): Ported to 0.9 using in place base tranform.
* gst/videomixer/Makefile.am:
* gst/videomixer/videomixer.c: (gst_videomixer_pad_get_type),
(gst_videomixer_pad_class_init),
(gst_videomixer_pad_sink_setcaps),
(gst_videomixer_pad_link), (gst_videomixer_pad_unlink),
(gst_videomixer_pad_init), (gst_videomixer_class_init),
(gst_videomixer_init), (gst_videomixer_getcaps),
(gst_videomixer_request_new_pad), (gst_videomixer_fill_queues),
(gst_videomixer_blend_buffers), (gst_videomixer_update_queues),
(gst_videomixer_collected), (gst_videomixer_change_state):
Ported
to 0.9 using collectpads.