libs/gst/base/gstbasesink.c: Latency query is allowed after we are prerolled. Introduce a new flag for this and stop ...

Original commit message from CVS:
* libs/gst/base/gstbasesink.c: (gst_base_sink_init),
(gst_base_sink_preroll_queue_flush), (gst_base_sink_commit_state),
(gst_base_sink_wait_preroll), (gst_base_sink_needs_preroll),
(gst_base_sink_set_flushing), (gst_base_sink_query),
(gst_base_sink_change_state):
Latency query is allowed after we are prerolled. Introduce a new flag
for this and stop abusing other variables.