It would be nice to have some low pass extension

There is an EFX extension for lowpass

https://github.com/kcat/openal-soft/blob/master/include/AL/efx.h

There are extensions for reverb and other extensions that are quite complex, so not sure about following efx extensions, but at least having some way to do a lowpass filter would be nice.

(maybe header should be `<AL/alext.h>` ?)

These would be really nice for in-game effects!