Allow prepass to run without ATTRIBUTE_NORMAL (#17881)

# Objective

Allow prepass to run without ATTRIBUTE_NORMAL. 
This is needed for custom materials with non-standard vertex attributes.
For example a voxel material with manually packed vertex data.

Fixes #13054.

This PR covers the first part of the **stale** PR #13569 to only focus
on fixing #13054.

## Solution

- Only push normals `vertex_attributes` when the layout contains
`Mesh::ATTRIBUTE_NORMAL`

## Testing

- Did you test these changes? If so, how?
**Tested the fix on my own project with a mesh without normal
attribute.**
- Are there any parts that need more testing?
  **I don't think so.**
- How can other people (reviewers) test your changes? Is there anything
specific they need to know?
**Prepass should not be blocked on a mesh without normal attributes
(with or without custom material).**
- If relevant, what platforms did you test these changes on, and are
there any important ones you can't test?
  **Probably irrelevant, but Windows/Vulkan.**

crates/bevy_pbr/src/prepass/mod.rs

@@ -507,8 +507,15 @@ where
             .mesh_key
             .intersects(MeshPipelineKey::NORMAL_PREPASS | MeshPipelineKey::DEFERRED_PREPASS)
         {
-            vertex_attributes.push(Mesh::ATTRIBUTE_NORMAL.at_shader_location(3));
             shader_defs.push("NORMAL_PREPASS_OR_DEFERRED_PREPASS".into());
+            if layout.0.contains(Mesh::ATTRIBUTE_NORMAL) {
+                shader_defs.push("VERTEX_NORMALS".into());
+                vertex_attributes.push(Mesh::ATTRIBUTE_NORMAL.at_shader_location(3));
+            } else if key.mesh_key.contains(MeshPipelineKey::NORMAL_PREPASS) {
+                warn!(
+                    "The default normal prepass expects the mesh to have vertex normal attributes."
+                );
+            }
             if layout.0.contains(Mesh::ATTRIBUTE_TANGENT) {
                 shader_defs.push("VERTEX_TANGENTS".into());
                 vertex_attributes.push(Mesh::ATTRIBUTE_TANGENT.at_shader_location(4));

crates/bevy_pbr/src/prepass/prepass.wgsl

@@ -96,6 +96,7 @@ fn vertex(vertex_no_morph: Vertex) -> VertexOutput {
 #endif // VERTEX_UVS_B
 
 #ifdef NORMAL_PREPASS_OR_DEFERRED_PREPASS
+#ifdef VERTEX_NORMALS
 #ifdef SKINNED
     out.world_normal = skinning::skin_normals(world_from_local, vertex.normal);
 #else // SKINNED
@@ -106,6 +107,7 @@ fn vertex(vertex_no_morph: Vertex) -> VertexOutput {
         vertex_no_morph.instance_index
     );
 #endif // SKINNED
+#endif // VERTEX_NORMALS
 
 #ifdef VERTEX_TANGENTS
     out.world_tangent = mesh_functions::mesh_tangent_local_to_world(

crates/bevy_pbr/src/prepass/prepass_io.wgsl

@@ -15,7 +15,9 @@ struct Vertex {
 #endif
 
 #ifdef NORMAL_PREPASS_OR_DEFERRED_PREPASS
+#ifdef VERTEX_NORMALS
     @location(3) normal: vec3<f32>,
+#endif
 #ifdef VERTEX_TANGENTS
     @location(4) tangent: vec4<f32>,
 #endif