setBfree does not work well with mixer

[riban-bw]

Describe the bug Adding a multi-channel setBfree configuration results in a range of issues with the mixer.

To Reproduce Steps to reproduce the behaviour:

1. Add setBfree - Hammond Emulator, MIDI Channel 1, Split Pedals + Lower + Upper, Sine, Blues 1

Expected behaviour SetBfree added as a single or multiple chains with corresponding mixer controls.

Actual behaviour

• SetBfree added as a single engine connected to mixer input 3
• Three chains are displayed in the mixer
• Mixer strips 1 & 2 controls have no effect
• Preset selection for Lower and Pedals is not complete
• Adding effect to strip 1 moves the chain to mixer channel 1
• Subsequently adding effect to strip 2 inserts the effect before effect on chain 1

Setup:

• HARDWARE Raspberry Pi 3 Model B Plus Rev 1.3 Audio: HifiBerry DAC+ ADC PRO Display: ZynScreen 3.5 (v1) Wiring: MCP23017_ZynScreen I2C: MCP23017@0x20 Profile: z2_v1
• SYSTEM Raspbian GNU/Linux 10 (buster) Build Date: 2021-09-23 Memory: 51% (477M/923M) SD Card: 60% (16G/29G) Temperature: 53.7ºC Overclock: Disabled
• MIDI Profile: default.sh Tuning: 440 Hz Receive Mode: Stage (Omni On) ZS3 Sub-SnapShots: Off MIDI to Output: On Master Channel: off
• SOFTWARE zyncoder: testing (e236925) zynthian-ui: testing (5a396b9) zynthian-sys: testing (15d5d4c) zynthian-data: testing (10e1f9f) zynthian-webconf: testing (09fbf91)

Additional context I have not necessarily documented all the side-effects but surfice to say that this is suboptimal behaviour. I don't think it is necessarily any worse than it has historically been so don't think it warrants fixing before 2205 release but we need to consider a solution. Maybe use of LV2 plugin as detailed by baggypants will offer a better implementation.

[jofemodo]

The plan for chainification is to flat all engines using several MIDI channels, so, for instance, setBfree engine will use a single mixer strip (it forms a single audio chain) but it would receive (listen to) several MIDI channels. The controllers for the different manuals will be accessible in a unified list of pages. Presets would be engine-wide, but you could have presets that only affect upper manual, etc. Something similar would be done for Aeolus.

Of course, improving the LV2 support for setBfree, as detailed by @baggypants is also an interesting way to follow, but i don't want to remove the standalone engine, as it have a number of advantages for organ players.

Regards,