libs/gst/base/gstbasesink.c: Improve position reporting in the flushing state.

libs/gst/base/gstbasesink.c: Improve position reporting in the flushing state.

Original commit message from CVS:
* libs/gst/base/gstbasesink.c: (gst_base_sink_render_object),
(gst_base_sink_event), (gst_base_sink_chain_unlocked),
(gst_base_sink_negotiate_pull), (gst_base_sink_pad_activate_pull),
(gst_base_sink_get_position), (gst_base_sink_change_state):
Improve position reporting in the flushing state.
Also report the position when we are not yet prerolled but we
have a newsegment event. Fixes #543444.
Improve the pull-based negotiation code.
* tests/check/elements/fakesink.c: (GST_START_TEST),
(fakesink_suite):
Add testcase for position reporting while flushing in PAUSED and
PLAYING.
* tests/check/generic/sinks.c: (GST_START_TEST):
Update unit-test, we can now query the position as soon as we receive a
NEWSEGMENT event.