Ears (music switching) seek

When switching music we don't seek the song but instead start from the beginning. It would be ideal if we could continue playing at the same spot where we were initially.
This'll probably require engine changes (https://xiph.org/vorbis/doc/vorbisfile/seeking.html at https://github.com/spring/spring/blob/develop/rts/System/Sound/OpenAL/OggStream.h and similar files)
