Audio format fixes

[rakslice]

• Use SDL_ConvertAudio to handle output audio conversion ** This takes care of 16-bit MSB format audio from the mac to LSB ** Also takes care of the case where when the mac is set to 8-bit stereo it sometimes outputs mono SoundComponentData when playing back a mono audio sample on the mac (e.g. stock alert sounds)
• Switch the 8-bit output format constant to AUDIO_U8 to avoid clicking due to the wrong silence values (this goes with making a change in sdl-1.2 to accept that)

Part of the fix for #7