Select/filter MIDI channel(s) for instrument plugins

TL;DR: have an option similar to the control channel assignment to select one or more MIDI channels an instrument plugin receives but for playback, ignoring all others. Of course leave an option for 'omni' (all channels) mode.

Motivation: currently most instrument plugins seem to work in omni mode by default (i.e. they will play anything coming on any MIDI channel). While this is great for with multi-instrument plugins (e.g. something like yoshimi or dexed), where you can assign each to its own channel, it's not always so great with 'single instrument' plugins like e.g. a drums plugin or something like MDA Piano, especially if being sent MIDI from a sequencer. In the sequencer I'd typically have all instruments in different tracks and use channels to assign instruments, trouble is that if I then send the sequencer-generated MIDI to Carla and route it to various 'single instrument' plugins I'll have all instrument play all channels. This can be worked around now either adding the internal MIDI Split plugin, but AFAIK this works only in Multiple client mode, or adding multiple outputs in the sequencer (the latter quite impractical if you use many instruments).

Ideally having this 'filtering' mechanism out of the box could be useful both for sequencing but also in live set-ups where e.g. you switch a channel on your MIDI keyboard (controller) to change instrument in Carla. Also in a hybrid setup where e.g. a sequencer is used as a 'backing track' and the user is playing live.

Example use case:

A user is sequencing in their sequencer and uses Carla to aggregate various plugins, including instrument ones
The user adds some 'single instrument' plugins (e.g. durms, piano, ...)
User MIDI-connects the sequencer to Carla and assigns in the sequencer specific channels to specific instruments
In Carla the user selects for each instrument the receiving channel(s)

falkTX / Carla

Select/filter MIDI channel(s) for instrument plugins #1408