update JUCE LV2 branch to the latest JUCE6 and enable VST3 build on Linux

[atsushieno]

It ist most unlikely ideal (because it is currently killing VeSTige based VST2 build), but based on jpcima/JUCE#5, this brings in VST3 support. Those massive changes are mostly for vst3sdk updates.

[jpcima]

Hi, does VST3 support handling distinct MIDI channels?
This plugin was always designed as multi-channel instrument, like Roland Sound Canvas.
At first I thought that VST3 takes control over MIDI channels in the host, and hence it's not possible to preserve the current model in this format. Is this actually not the case?

[atsushieno]

That is an interesting concern, I hadn't thought of that. Is it something like this? https://forum.juce.com/t/what-is-the-intention-of-macro-juce-vst3-emulate-midi-cc-with-parameters/17858/13

I tried this with Carla and my own virtual MIDI keyboard app to confirm that the ALSA MIDI inputs on different channels are successfully sent to OPNplug. I recorded the behaviors using OBS studio.

While notes are successfully sent (2020-10-10 00-11-50.zip), program changes aren't
(2020-10-10 00-26-49.zip). So your concern is correct, it's not fully compatible.

[atsushieno]

Just noticed that there is an update to JUCE6 in the main branch. There is no need for this PR then.