gst/videomixer/videomixer.c: Be a nicer negotiation citizen and provide a getcaps function on the srcpad. This also f...

Original commit message from CVS:
* gst/videomixer/videomixer.c: (gst_videomixer_pad_get_type),
(gst_videomixer_pad_class_init), (gst_videomixer_pad_get_property),
(gst_videomixer_pad_set_property),
(gst_videomixer_pad_sinkconnect), (gst_videomixer_pad_init),
(gst_video_mixer_background_get_type), (gst_videomixer_get_type),
(gst_videomixer_class_init), (gst_videomixer_init),
(gst_videomixer_getcaps), (gst_videomixer_request_new_pad),
(gst_videomixer_blend_ayuv_i420), (pad_zorder_compare),
(gst_videomixer_sort_pads), (gst_videomixer_fill_checker),
(gst_videomixer_fill_color), (gst_videomixer_fill_queues),
(gst_videomixer_blend_buffers), (gst_videomixer_update_queues),
(gst_videomixer_loop), (plugin_init):
Be a nicer negotiation citizen and provide a getcaps function on
the srcpad. This also fixes a crash when resizing.