Fix wesl in wasm and webgl2 (#18591)

# Objective

- feature `shader_format_wesl` doesn't compile in Wasm
- once fixed, example `shader_material_wesl` doesn't work in WebGL2

## Solution

- remove special path handling when loading shaders. this seems like a
way to escape the asset folder which we don't want to allow, and can't
compile on android or wasm, and can't work on iOS (filesystem is rooted
there)
- pad material so that it's 16 bits. I couldn't get conditional
compilation to work in wesl for type declaration, it fails to parse
- the shader renders the color `(0.0, 0.0, 0.0, 0.0)` when it's not a
polka dot. this renders as black on WebGPU/metal/..., and white on
WebGL2. change it to `(0.0, 0.0, 0.0, 1.0)` so that it's black
everywhere

assets/shaders/custom_material.wesl

@@ -6,7 +6,8 @@ struct VertexOutput {
 }
 
 struct CustomMaterial {
-    time: f32,
+    // Needed for 16-bit alignment on WebGL2
+    time: vec4<f32>,
 }
 
 @group(2) @binding(0) var<uniform> material: CustomMaterial;
@@ -15,5 +16,5 @@ struct CustomMaterial {
 fn fragment(
     mesh: VertexOutput,
 ) -> @location(0) vec4<f32> {
-    return make_polka_dots(mesh.uv, material.time);
-}
+    return make_polka_dots(mesh.uv, material.time.x);
+}

assets/shaders/util.wesl

@@ -40,5 +40,5 @@ fn make_polka_dots(pos: vec2<f32>, time: f32) -> vec4<f32> {
         is_dot = step(dist_from_center, 0.3 + wave_normalized * 0.2);
     }
 
-    return vec4<f32>(dot_color * is_dot, is_dot);
-}
+    return vec4<f32>(dot_color * is_dot, 1.0);
+}

crates/bevy_render/src/render_resource/shader.rs

@@ -163,26 +163,8 @@ impl Shader {
 
         match import_path {
             ShaderImport::AssetPath(asset_path) => {
-                let asset_path = std::path::PathBuf::from(&asset_path);
-
-                // Get the base path and canonicalize it to match the format of the asset path
-                let mut base_path = bevy_asset::io::file::FileAssetReader::get_base_path();
-                base_path.push("assets");
-                let base_path = base_path.canonicalize().unwrap_or(base_path);
-
-                // Try to make the path relative to the base path
-                let relative_path = match asset_path.canonicalize() {
-                    Ok(canonical_asset_path) => {
-                        match canonical_asset_path.strip_prefix(&base_path) {
-                            Ok(rel_path) => rel_path.to_path_buf(),
-                            Err(_) => canonical_asset_path,
-                        }
-                    }
-                    Err(_) => asset_path,
-                };
-
                 // Create the shader import path - always starting with "/"
-                let shader_path = std::path::Path::new("/").join(&relative_path);
+                let shader_path = std::path::Path::new("/").join(&asset_path);
 
                 // Convert to a string with forward slashes and without extension
                 let import_path_str = shader_path

examples/shader/shader_material_wesl.rs

@@ -59,7 +59,7 @@ fn setup(
     commands.spawn((
         Mesh3d(meshes.add(Cuboid::default())),
         MeshMaterial3d(materials.add(CustomMaterial {
-            time: 0.0,
+            time: Vec4::ZERO,
             party_mode: false,
         })),
         Transform::from_xyz(0.0, 0.5, 0.0),
@@ -80,7 +80,7 @@ fn update(
 ) {
     for (material, mut transform) in query.iter_mut() {
         let material = materials.get_mut(material).unwrap();
-        material.time = time.elapsed_secs();
+        material.time.x = time.elapsed_secs();
         if keys.just_pressed(KeyCode::Space) {
             material.party_mode = !material.party_mode;
         }
@@ -95,8 +95,9 @@ fn update(
 #[derive(Asset, TypePath, AsBindGroup, Clone)]
 #[bind_group_data(CustomMaterialKey)]
 struct CustomMaterial {
+    // Needed for 16 bit alignment in WebGL2
     #[uniform(0)]
-    time: f32,
+    time: Vec4,
     party_mode: bool,
 }