sys/xvimage/xvimagesink.c: Protect interface methods from chain and negotiation and vice versa (Fixes #166142).

Original commit message from CVS:
2005-02-11  Julien MOUTTE  <julien@moutte.net>

* sys/xvimage/xvimagesink.c: (gst_xvimagesink_xvimage_put),
(gst_xvimagesink_sink_link), (gst_xvimagesink_change_state),
(gst_xvimagesink_chain), (gst_xvimagesink_buffer_free),
(gst_xvimagesink_buffer_alloc), (gst_xvimagesink_set_xwindow_id),
(gst_xvimagesink_expose), (gst_xvimagesink_set_property),
(gst_xvimagesink_finalize), (gst_xvimagesink_init): Protect interface
methods from chain and negotiation and vice versa (Fixes #166142).
Fix a possible bug of images in the buffer pool being discarded because
we are looking at the wrong geometry.
* sys/xvimage/xvimagesink.h: Add stream_lock.