gst/videomixer/videomixer.c: Avoid divide by zero, choose masterpad as the pad with the highest framerate.

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_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):
Avoid divide by zero, choose masterpad as the pad with the highest
framerate.