Provide feature flags for high-level engine components

[TitanNano]

I'm aware that this might introduce too much complexity to be maintainable, but I wanted to raise the idea in case it could be feasible.

It would be useful to introduce feature flags for high-level engine features so we can customize the feature set at build time. All features would remain enabled by default. I'm thinking of features like graphics, audio, physics, 3d and 2d. This could reduce build time and target binary size when a game does not require some of these features.

Some of my use cases would be:

• Game server that only requires 2D physics, no graphics, no 3D, no audio. There is no need to compile 3D physics, graphics crates, or audio crates. It would also eliminate the need to dynamically link against ALSA audio.
• Pure 2D game, only requires 2D physics, no 3D models, no 3D physics

Some additional hypothetical use cases:

• Pure 3D game, the inverse of 2D, no 2D tilemaps or sprites, no 2D physics
• none-game application without sound; no need for audio dependencies.

I can completely understand if introducing multiple feature flags like this might add too much complexity to the project.

Note

I found #194 but I'm not entirely sure how much things have changed since then.

[mrDIMAS]

I think that these features can disable respective backends, but leave scene nodes intact. This is the easiest way of implementing feature flags. For example, audio feature flag could just make fyrox-sound dependency optional and this would simply mean that there will be no sound, but Sound and Listener scene nodes can still be used. I think I'll try to implement this after the release of 1.0