This function was intended to work around a certain class of bugs
inside video decoders whereby the a reference picture was never
dereferenced, i.e. missing decoder_UnlinkPicture() call.

There are however some situations where this hack would release a
still referenced picture:

- If the video output or decoder has a high latency or if a video
  filter holds pictures (deinterlace), there may be zero free pictures
  even though the total number of pictures is sufficient overall.
  In that case, waiting for a picture buffer to be released normally
  is the right and safe approach.

- If the byte stream is invalid or corrupt such that the number of
  required pictures (DPB size) is underestimated, dropping frames is
  acceptable. Frames would be corrupt anyway due to missing references.
  (This case could be better worked around by allocating extra
   pictures on-the-fly, though this would require memory copying in the
   video output.)

- Even if the decoder indeed leaks pictures, the oldest referenced
  picture is not necessarily among those leaked.

If the picture was not truly leaked, vout_FixLeaks() would cause an
extraneous picture release. This should lead to an assertion failure in
picture_Release(). Without assertions, it would lead to undefined
behaviour, especially invalid pointer use in case of hardware surfaces.

In any case, picture leaks are still recovered when resetting the video
output with vout_Reset(), after the decoder is destroyed. That might
turn out to be a problem too though.