oggdec: Replace stream start delay by minimum input queue size.

Waiting 300ms at stream start is both ugly and unnecessary if playback
does not start in the middle of the stream. This patch removes the
->stream_start member of struct private_oggdec_data and delays plaback
only if ov_read returns OV_HOLE, i.e. if streaming does not start at
the beginning of the file.

This delay is enforced by setting the minimal input queue size.