Change Texture2D::fill to be non-mutable

[maxime-tournier]

This PR changes Texture2D::fill to be non-mutable, so that one can change a texture's contents through a material Texture2DRef (which is behind an Arc). I'm using this to display my webcam stream on a textured quad.

It's unclear to me why the method currently takes a mutable ref to self since all the side-effects are on the GPU (just like generate_mip_maps, which is non-mutable), so feel free to enlighten me on this point :)

Best regards,

[asny]

Sorry for the really really late reply 😬 I think I was just being a bit idealistic and wanted functions that mutated something to be mutable so you don't accidentally edit something and it follows the Rust ideology. I think it's a nice idea, but it's hard to do in practice. For example, the render targets are usually used for writing, hence it's mutable to create a render target, but you can also use them for reading back to the CPU, so in that case it should be non-mutable. And your example with generating mip maps should also take a mutable reference, but since it's not enforced by the compiler, I didn't see it.

So I think I'm going to change all cases of &mut self to &self in the core to be consistent and not limit anybody. Finally, I think the real problem is the TextureRef, it should not need an Arc, but that's another problem 🙂

[asny]

Changed the rest of the texture functions here

[maxime-tournier]

Thanks a lot for the detailed explanations 👍