gst/gstpad.c: check that pads that get linked belong to the same manager. The old...

gst/gstpad.c: check that pads that get linked belong to the same manager. The old code allowed linking elements befor...

Original commit message from CVS:
* gst/gstpad.c: (gst_pad_check_schedulers),
(gst_pad_can_link_filtered), (gst_pad_link_filtered),
(gst_pad_link_prepare):
check that pads that get linked belong to the same manager. The old
code allowed linking elements before putting them into bins, so it
worked to link them and then put them in different threads, which
lead to weird behaviour.
Since this effectively disallows linking elements before putting
them in a bin, some applications might not work after this and error
out. If these applications are too critical, we might need to revert
that patch. Please test this before the next release...