Bring back volume display
Subtract average filter again on audio output

Subtract a moving average to avoid clicks.
Clamp number of samples to next power-of-two

This is required by the SDL API
(see http://wiki.libsdl.org/SDL_AudioSpec)
Improve UI cleanup on Linux

Clear to base foreground/background instead of 7.
Fix twofold error in AudioWait/AudioCallback

- obtained_samples is in complete samples (so r and l), needs to be
multiplied with two to derive the buffer size.
- don't call SDL_[Un]LockAudio() in the callback. It's meant to avoid the
callback.