Also, get rid of the canvas's ability to display different shapes on the same
canvas, since we never use that feature anyway.

Also, use Channel 0 CC 20 0-127 for pattern triggering instead of NOTE ONs.

Pattern muteing/unmuteing (via GUI or MIDI) takes effect the next time the
pattern loops.

Notes 1-128 on Channel 1 correspond to patterns. NOTE ON triggers the pattern
at the given moment, a second NOTE ON stops it at a given moment. NOTE OFF does
nothing. Also, patterns triggered via the GUI are no longer forced to be in
lock-step.

Now patterns should be triggered properly even if we come in in the
middle of a phrase, as happens when randomly relocating the transport.

that resulted in all non-note events being dropped.