gst/gstsegment.c: Also accumulate time correctly when doing reverse playback. Fixes...

gst/gstsegment.c: Also accumulate time correctly when doing reverse playback. Fixes #488201,

Original commit message from CVS:
* gst/gstsegment.c: (gst_segment_set_newsegment_full),
(gst_segment_to_stream_time), (gst_segment_to_running_time):
Also accumulate time correctly when doing reverse playback. Fixes
#488201,
When converting to running and stream time, use default values for
start/stop/time/accum when comparing different formats. Fixes #494245.
* libs/gst/base/gstbasesink.c: (gst_base_sink_get_sync_times):
Do running/stream time in TIME format.
* tests/check/gst/gstsegment.c: (GST_START_TEST),
(gst_segment_suite):
2 new unit tests for segment accumulation.