Polymeter 0.0.21 adds MIDI Output and Piano bars

Polymeter 0.0.21 adds two dynamic visualizations that can help you grasp what you're hearing: a MIDI output bar that shows the output MIDI events as text, with filtering options, and a piano bar that displays the output notes on a piano keyboard, also with filtering. Both of these visualizations were proposed in a previous post.

The main issue with implementing visualizations is a multithreaded design problem, which stems from the fact that the sequencer runs in a different thread from the rest of the application (the sequencer callback thread). In order to safely transfer data from one thread to another, serialization is required, but the goal is to minimize the likelihood of the sequencer callback thread ever being blocked by the user interface thread. The most appropriate and performant architecture for this situation is a buffer swapping scheme. In order to reduce time spent in the critical section to the absolute minimum, only pointers to the buffers are exchanged, rather than the buffers themselves. Fortunately the dynamic array class already supports swapping buffer pointers with another array instance. The result is the best of both worlds: zero-copy exchange between a pair of growable arrays.

This version also adds a command to reset the window layout to defaults. Thanks to Benoit Ruelle for suggesting this useful feature.