Allow rendering meshes without UV coordinate data. (#5222)

# Objective

Bevy requires meshes to include UV coordinates, even if the material does not use any textures, and will fail with an error `ERROR bevy_pbr::material: Mesh is missing requested attribute: Vertex_Uv (MeshVertexAttributeId(2), pipeline type: Some("bevy_pbr::material::MaterialPipeline<bevy_pbr::pbr_material::StandardMaterial>"))` otherwise. The objective of this PR is to permit this.

## Solution

This PR follows the design of #4528, which added support for per-vertex colours. It adds a shader define called VERTEX_UVS which indicates the presence of UV coordinates to the shader.

src/loader.rs

@@ -260,11 +260,6 @@ async fn load_gltf<'a, 'b>(
                 .map(|v| VertexAttributeValues::Float32x2(v.into_f32().collect()))
             {
                 mesh.insert_attribute(Mesh::ATTRIBUTE_UV_0, vertex_attribute);
-            } else {
-                let len = mesh.count_vertices();
-                let uvs = vec![[0.0, 0.0]; len];
-                bevy_log::debug!("missing `TEXCOORD_0` vertex attribute, loading zeroed out UVs");
-                mesh.insert_attribute(Mesh::ATTRIBUTE_UV_0, uvs);
             }
 
             if let Some(vertex_attribute) = reader