sys/ximage/ximagesink.c: Protect interface methods from chain and negotiation and...

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

Original commit message from CVS:
2005-02-12  Julien MOUTTE  <julien@moutte.net>

* sys/ximage/ximagesink.c: (gst_ximagesink_xwindow_new),
(gst_ximagesink_sink_link), (gst_ximagesink_change_state),
(gst_ximagesink_chain), (gst_ximagesink_set_xwindow_id),
(gst_ximagesink_expose), (gst_ximagesink_set_property),
(gst_ximagesink_finalize), (gst_ximagesink_init): Protect interface
methods from chain and negotiation and vice versa (Fixes #166142).
* sys/ximage/ximagesink.h: Add stream_lock.
* sys/xvimage/xvimagesink.c: (gst_xvimagesink_sink_link),
(gst_xvimagesink_chain), (gst_xvimagesink_buffer_free),
(gst_xvimagesink_buffer_alloc), (gst_xvimagesink_set_xwindow_id),
(gst_xvimagesink_expose): Check for xcontext before trying to link.