organelle: Implement volume control and metering

[ssssam]

Key 6 should control output level. mother.pd handles this for PD patches - the 5th knob value is sent to vol.

Level meters should update - in mother.pd it looks like this:

[ssssam]

Outputs of that patch are 5 ints sent to /oled/vumeter. First 4 are meter values for in L/R, out L/R. 5th value is 'peaks' - seems to be bitflags where lowest 4 bits are true to represent a peak on each channel.

[ssssam]

Other metering code:

• https://github.com/x42/meters.lv2/blob/master/src/meters.cc
• https://docs.rs/dasp/0.11.0/dasp/rms/index.html, https://docs.rs/dasp/0.11.0/dasp/peak/index.html, https://docs.rs/dasp/0.11.0/dasp/envelope/index.html