-removed redundant fseeks for WAV, there's an fseek right before the fread, only pos needs to be set (#161)

-added proper seek and tell for OGG
-fixed WAV seek and tell to Lua to be in num samples instead of byte count
-fix for when play is called on a source again before it finished playing, to match Love2D, pos should not be reset. Probably also helps if we implement Pause, since also then Play should leave pos where it is.
-pos now consistently reset on stop