[Feature request] Support for OpenAL backend

[s-d-m]

The audio backend "natively" supported by webassembly is OpenAL, which fluidsynth does not support at the moment. Adding it would make using fluidsynth on web applications easy.

Current alternatives are:
1/ trying to use SDL audio as fluidsynth backend, and hook SDL audio output to webassembly but that makes for a complicated setup.
2/ use fluid_synth_write_s16 to get audio data from fluidsynth into a buffer, and then push that buffer to openAL. You can see this in action here https://gist.github.com/take-cheeze/43f026d07547b9399738 or in my personal toy project at https://github.com/s-d-m/lilyplayer/blob/dba2dfd5484c71b5de49a96e9a18e2d8776f5c1c/src/sound_player.cc#L89-L114

To make fluidsynth available to webassembly, on top of OpenAl support, the changes at https://github.com/jet2jet/fluidsynth-emscripten are also useful.

[derselbst]

You're welcome to file a PR for that. I currently don't have time for it.

[s-d-m]

Reading a bit into how openAL works (I admittedly didn't extensively went through it), it looks to me that the proper way to do it is by using the alternative #2 I wrote above. Unless someone with better understanding of openAL can come and correct me here, I think this feature request is a non-issue and can be closed.

[ThreeDeeJay]

OpenAL (Soft) used to have MIDI support so perhaps there's anything useful in the old code that was removed: kcat/openal-soft@6689c61
I for one would also like FluidSynth to support OpenAL, but for a different reason: (3D) spatial audio and (EAX) reverb.
If every instrument (or even note) was treated as an OpenAL sound emitter/object, it could be spatialized using attributes like note balance and pitch for the X and Y coordinates or something like this https://youtu.be/h8og1oL8JVY or https://youtu.be/EmuWIuI9e-c