Godot virtuals are no longer required if a derived Godot class implements them

[Bromeon]

Closes #1133.

[GodotRust]

API docs are being generated and will be shortly available at: https://godot-rust.github.io/docs/gdext/pr-1136

[djcsdy]

This might be considered only a partial fix.

AIUI, after this change, mix is no longer required, which is definitely an improvement.

However in practice, if the programmer does provide an implementation of mix, Godot will never call it.

The reason seems to be that AudioStreamPlaybackResampled.mix is not marked GDVIRTUAL: https://github.com/godotengine/godot/blob/4.4/servers/audio/audio_stream.h#L154

I assume this means that although it is a virtual method in the C++ code, it is not treated as virtual from the point of view of extensions.

It's not clear if that's deliberate but it would make sense.

So if possible it might be better to not only make mix optional but remove it from the trait altogether.

[Bromeon]

Thanks for the feedback. Good call, I asked on RocketChat to clarify the semantics.

If that's by design, then I agree, we should remove such methods from the trait. It's a bit strange from LSP perspective, but now our design choice to have one I* trait, rather than one per base class, will be rewarded 🙂