CLAP: add option to set supported dialect (e.g. CLAP_NOTE_DIALECT_MIDI_MPE)

[jfrey-xx]

Hello,

Fist of all thank you for this framework, I'm creating my first audio plugins with it, and it's a breeze to get working my various synths / effects / utilities, with so many formats at once :)

I've been testing my plugins with the newly released Bitwig 5.1 (in Demo mode) and at the same time I wanted to try some things with pitch bend. I realized first that with 5.1 Bitwig now seems to acknowledge the CLAP_NOTE_DIALECT_MIDI flag, e.g. the "note on" events are not anymore processed through CLAP_EVENT_NOTE_ON in DistrhoPluginCLAP.cpp, but as regular MIDI events. The current workaround might probably stick for other cases. (I'll just refer to #383 (comment) if that can help to sort things).

The real issue I faced is that with 5.1 the pitch bend is not sent by bitwig anymore to my clap plugins (e.g. using the "fine" knob of the "note transpose" Note FX). For some reason I had to change at least the "supported" dialect flag in DistrhoPluginCLAP.cpp, from CLAP_NOTE_DIALECT_MIDI to CLAP_NOTE_DIALECT_MIDI_MPE. I guess this follows (odd?) Bitwig's behavior for the other formats (VST2 & VST3), where pitch bend is only sent if "MPE" is selected in the inspector. At my level I don't see any side effects, I would think that letting people choose the dialect flags offer more features to developers but I don't have the full picture.

To have the pitch bend perfectly usable in Bitwig I should also find a way to enable the selection of the pitch bend range in the DAW. There is no such option in inspector as compared to the VST plugins, and the host seems to default to +/- 48 semitones. I will handle that with a parameter at the plugin level, a good idea anyhow.

I spoke only of pitch bend, there might be other parameters (aftertouch?) that might be enabled in Bitwig through this MPE flag, I can do more testing if necessary.