Headless rendering to images

[cart]

The RenderGraph should already be able to handle headless rendering scenarios. The only missing pieces are:

• Adapt WindowTextureNode to a TextureNode, which produces a texture output for configured dimensions instead of window dimensions
• Write code to copy texture data from the gpu to an image file so headless rendering can be verified

[rmsc]

It definitely does: https://github.com/mrk-its/bevy/tree/render_to_texture and https://github.com/rmsc/bevy/tree/render_to_file.

There are however a few open questions, namely:

• The Camera system expects a WindowId. Maybe change that to an enum to also support a TextureId?
• There is a Res<Windows> for WindowId-to-Window mapping, perhaps there should also be one for TextureId-to-Texture?
• To me it's not clear how to properly interface with the ECS to move the texture from the GPU to the CPU (possibly also related to Distinguish between "GPU textures" and "CPU textures" #1234)

[alice-i-cecile]

Complete! See #1927.

[mockersf]

Complete! See #1927.

Not really. It's not possible to render to an image file as the issue was saying

[DGriffin91]

I made a little example of this awhile back. If it would be helpful, I could update it.

Write code to copy texture data from the gpu to an image file so headless rendering can be verified

Are we looking for an example or a test here?