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.
Add out of range test for tone

Note 71 of the percussion of bank 31 was crashing the synth.
An integer overflow combined with exponentiation resulted in such a large
value that it is regarded as infinite. Code flow was stuck in
the hz() loop to determine the octave.
Add appropriate range checks to avoid this in the future.
Fix number of lines in UI::PrintLn
Only show portamento warning if !=0

This avoids a flood of warnings when resetting controllers.
Revert 64c20ce (Start audio after first samples)

Solve this in audioout.cc, not in each executable.
Handle audio start/unpause automatically for SDL
Implement `all sounds off` (controller 120)
Warn when playing missing instrument

Mark instruments with peak amplitude <0.5 as having no sound.
Log a warning when one of these is played.
Show name as instrument bank as well as number

At startup show the name of the selected instrument bank as well.
Fix off-by-one error in MIDIsymbols
Add missing drums to MIDIsymbols

Just add 'D' glyphs for now.

78 Mute Cuica
79 Open Cuica
80 Mute Triangle
81 Open Triangle
Add GS percussion instruments
gen_adldata: Replace missing instruments with 0
gen_adldata: Avoid conflicts between melodic and drum banks

In some of the bnk files (eg: Int) the percussive and non-percussive
instruments are detected wrongly resulting in the melodic bank
being overwritten by drum instruments. I noticed this while listening
the Descent level 3 music with the accompanying intmelo bank.
Pass the percussion flag into LoadBNK explicitly.
instruments.py: 256-color experiments
Add "legend" to specify what instrument is playing on what channel