imagefreeze: clear 0 DTS on buffers output, as sinks will prefer DTS over PTS for syncing
Since the initial decoded still image buffer will have dts=pts=0, and we only set PTS on buffers we push out, all buffers pushed out would have a DTS of 0. Sinks, however, will prefer DTS over PTS if both are set, and will therefore always see a timestamp of 0 no matter what the PTS is set to. Fixes unit test too.
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