plugins/elements/: Fix queue negotiation. If acceptcaps unconditionally returns TRUE...

plugins/elements/: Fix queue negotiation. If acceptcaps unconditionally returns TRUE, upstream is tricked into thinki...

Original commit message from CVS:
* plugins/elements/gstmultiqueue.c: (gst_single_queue_push_one),
(gst_single_queue_new):
* plugins/elements/gstqueue.c: (gst_queue_init),
(gst_queue_push_one):
Fix queue negotiation. If acceptcaps unconditionally returns TRUE,
upstream is tricked into thinking it can suggest a format downstream
while downstream does not support that format. The real problem is that
core calls acceptcaps when pushing a buffer with new caps, for which we
do a little workaround by setting the caps on the srcpad ourselves
before pushing the buffer (until this is figured out). Fixes #486758.