Objective

• Users often want to run a fullscreen shader but the current solution involves copying the custom_post_processing example which is a 350 line file with a lot of low level wgpu complexity. Users shouldn't have to deal with that just to make a fullscreen shader

Solution

• Introduce a new FullscreenMaterial trait and FullscsreenMaterialPlugin
• This new material will run a fullscreen triangle with the specified shader. It builds on top of the existing FullscreenShader infrastructure
• It lets user customize the node ordering. There's no defaults right now becausae it's intended as a bit of a primitive plugin. Eventually we could have some kind of default for custom post processing

Testing

Made a new fullscreen_material example and made sure it works

Follow up

Once this is merged there are various things that should be done to improve it. Add the option to bind the depth texture, offer defaults for post processing, use a full AsBindGroup, add a way to bind the gbuffer.