qtdemux: Initialize QtDemuxStream.segment in its constructor

This patch moves the initialization of QtDemuxStream.segment from gst_qtdemux_add_stream() to _create_stream(). This ensures the segment is always initialized when the stream is created.

Otherwise the segment format is left as GST_FORMAT_UNDEFINED in the case were a track is reparsed and qtdemux_reuse_and_configure_stream() is called instead of gst_qtdemux_add_stream(). (See qtdemux_expose_streams() in the non streams-aware case.)

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