Support for Raw GpuImage in AsBindGroup Derive Macro

[couyit]

I would like to request the addition of support for raw GpuImage in the AsBindGroup derive macro.

Currently, when an Image is added as an asset, it is automatically extracted into the RenderWorld and converted into a GpuImage by the ImagePlugin, making it impossible to reference an already created GpuImage as a binding.

This feature would allow for the binding of pre-existing wgpu textures. Additionally, it could be used to asynchronously execute compute shaders and display their results.

Example:

#[derive(AsBindGroup)]
struct MyMaterial {
    #[rawtexture(0)]
    my_texture: GpuImage,
}

[couyit]

I have created an initial implementation of this feature and was able to run it in my own project.
Here is the implementation. https://github.com/couyit/bevy/blob/main/crates/bevy_render/macros/src/as_bind_group.rs

Example:

#[derive(AsBindGroup)]
struct MyMaterial {
    // By adding "raw", it indicates the use of GpuImage.
    // I thought it would be better to use an existing attribute rather than adding a custom one.
    #[texture(0, raw)]
    #[sampler(1)]
    my_texture: GpuImage,
}

[couyit]

I realized that this issue can be resolved without modifying Bevy's source code. By writing the results calculated in the ComputeShader to a Texture and copying it to a GpuImage in the Render frame, the same functionality can be reproduced.