replaygain: allow exporting already decoded replaygain values

This adds a function to export raw replaygain values (i.e. in the (u)int32_t
form). It first checks whether AV_PKT_DATA_REPLAYGAIN side data is present, in
which case it does nothing.

Signed-off-by: Anton Khirnov <anton@khirnov.net>
