Added SDL_CloseAudio to APU.cpp:init_audio()

- Added SDL_CloseAudio to APU.cpp:init_audio() to make sure the audio
device is closed before trying to reopen it.
- Added SDL_GetError print statement because SDL has been reporting an
error this entire time and I didn't know.
- Added Audio_pause when pausing emulation
- Added auto-exit fullscreen when opening a rom while in fullscreen