INTERFACE=sdl
Change mixer panning to make sense

This makes positional audio in sdl work correctly (things to the left
play sounds in the left speaker)
Merge remote-tracking branch 'Furyhunter/sdl-fix-flippedchannels'
 Change mixer panning to make sense (for SDL12)

    This makes positional audio in sdl work correctly (things to the left
    play sounds in the left speaker)