Make AsBindGroup unsized (#6937)

# Objective

`AsBindGroup` can't be used as a trait object because of the constraint `Sized` and because of the associated function.

This is a problem for [`bevy_atmosphere`](https://github.com/JonahPlusPlus/bevy_atmosphere) because it needs to use a trait that depends on `AsBindGroup` as a trait object, for switching out different shaders at runtime. The current solution it employs is reimplementing the trait and derive macro into that trait, instead of constraining to `AsBindGroup`.

## Solution

Remove the `Sized` constraint from `AsBindGroup` and add the constraint `where Self: Sized` to the associated function `bind_group_layout`. Also change `PreparedBindGroup<T: AsBindGroup>` to `PreparedBindGroup<T>` and use it as `PreparedBindGroup<Self::Data>` instead of `PreparedBindGroup<Self>`.

This weakens the constraints, but increases the flexibility of `AsBindGroup`.
I'm not entirely sure why the `Sized` constraint was there, because it worked fine without it (maybe @cart wasn't aware of use cases for `AsBindGroup` as a trait object or this was just leftover from legacy code?).

---

## Changelog

- `AsBindGroup` can be used as a trait object.

crates/bevy_render/macros/src/as_bind_group.rs

@@ -371,7 +371,7 @@ pub fn derive_as_bind_group(ast: syn::DeriveInput) -> Result<TokenStream> {
                 render_device: &#render_path::renderer::RenderDevice,
                 images: &#render_path::render_asset::RenderAssets<#render_path::texture::Image>,
                 fallback_image: &#render_path::texture::FallbackImage,
-            ) -> Result<#render_path::render_resource::PreparedBindGroup<Self>, #render_path::render_resource::AsBindGroupError> {
+            ) -> Result<#render_path::render_resource::PreparedBindGroup<Self::Data>, #render_path::render_resource::AsBindGroupError> {
                 let bindings = vec![#(#binding_impls,)*];
 
                 let bind_group = {

crates/bevy_render/src/render_resource/bind_group.rs

@@ -253,7 +253,7 @@ impl Deref for BindGroup {
 ///     }
 /// }
 /// ```
-pub trait AsBindGroup: Sized {
+pub trait AsBindGroup {
     /// Data that will be stored alongside the "prepared" bind group.
     type Data: Send + Sync;
 
@@ -264,10 +264,12 @@ pub trait AsBindGroup: Sized {
         render_device: &RenderDevice,
         images: &RenderAssets<Image>,
         fallback_image: &FallbackImage,
-    ) -> Result<PreparedBindGroup<Self>, AsBindGroupError>;
+    ) -> Result<PreparedBindGroup<Self::Data>, AsBindGroupError>;
 
     /// Creates the bind group layout matching all bind groups returned by [`AsBindGroup::as_bind_group`]
-    fn bind_group_layout(render_device: &RenderDevice) -> BindGroupLayout;
+    fn bind_group_layout(render_device: &RenderDevice) -> BindGroupLayout
+    where
+        Self: Sized;
 }
 
 /// An error that occurs during [`AsBindGroup::as_bind_group`] calls.
@@ -277,10 +279,10 @@ pub enum AsBindGroupError {
 }
 
 /// A prepared bind group returned as a result of [`AsBindGroup::as_bind_group`].
-pub struct PreparedBindGroup<T: AsBindGroup> {
+pub struct PreparedBindGroup<T> {
     pub bindings: Vec<OwnedBindingResource>,
     pub bind_group: BindGroup,
-    pub data: T::Data,
+    pub data: T,
 }
 
 /// An owned binding resource of any type (ex: a [`Buffer`], [`TextureView`], etc).

examples/3d/skybox.rs

@@ -227,7 +227,7 @@ impl AsBindGroup for CubemapMaterial {
         render_device: &RenderDevice,
         images: &RenderAssets<Image>,
         _fallback_image: &FallbackImage,
-    ) -> Result<PreparedBindGroup<Self>, AsBindGroupError> {
+    ) -> Result<PreparedBindGroup<Self::Data>, AsBindGroupError> {
         let base_color_texture = self
             .base_color_texture
             .as_ref()