Add example showing how to use SpecializedMeshPipeline (#14370)

# Objective

- A lot of mid-level rendering apis are hard to figure out because they
don't have any examples
- SpecializedMeshPipeline can be really useful in some cases when you
want more flexibility than a Material without having to go to low level
apis.

## Solution

- Add an example showing how to make a custom `SpecializedMeshPipeline`.

## Testing

- Did you test these changes? If so, how?
- Are there any parts that need more testing?
- How can other people (reviewers) test your changes? Is there anything
specific they need to know?
- If relevant, what platforms did you test these changes on, and are
there any important ones you can't test?

---

## Showcase

The examples just spawns 3 triangles in a triangle pattern.


![image](https://github.com/user-attachments/assets/c3098758-94c4-4775-95e5-1d7c7fb9eb86)

---------

Co-authored-by: Alice Cecile <[email protected]>

Author: IceSentry

Cargo.toml

@@ -2394,6 +2394,17 @@ description = "A shader that shows how to bind and sample multiple textures as a
 category = "Shaders"
 wasm = false
 
+[[example]]
+name = "specialized_mesh_pipeline"
+path = "examples/shader/specialized_mesh_pipeline.rs"
+doc-scrape-examples = true
+
+[package.metadata.example.specialized_mesh_pipeline]
+name = "Specialized Mesh Pipeline"
+description = "Demonstrates how to write a specialized mesh pipeline"
+category = "Shaders"
+wasm = true
+
 # Stress tests
 [[package.metadata.example_category]]
 name = "Stress Tests"

assets/shaders/specialized_mesh_pipeline.wgsl

@@ -0,0 +1,48 @@
+//! Very simple shader used to demonstrate how to get the world position and pass data
+//! between the vertex and fragment shader. Also shows the custom vertex layout.
+
+// First we import everything we need from bevy_pbr
+// A 2d shader would be vevry similar but import from bevy_sprite instead
+#import bevy_pbr::{
+    mesh_functions,
+    view_transformations::position_world_to_clip
+}
+
+struct Vertex {
+    // This is needed if you are using batching and/or gpu preprocessing
+    // It's a built in so you don't need to define it in the vertex layout
+    @builtin(instance_index) instance_index: u32,
+    // Like we defined for the vertex layout
+    // position is at location 0
+    @location(0) position: vec3<f32>,
+    // and color at location 1
+    @location(1) color: vec4<f32>,
+};
+
+// This is the output of the vertex shader and we also use it as the input for the fragment shader
+struct VertexOutput {
+    @builtin(position) clip_position: vec4<f32>,
+    @location(0) world_position: vec4<f32>,
+    @location(1) color: vec3<f32>,
+};
+
+@vertex
+fn vertex(vertex: Vertex) -> VertexOutput {
+    var out: VertexOutput;
+    // This is how bevy computes the world position
+    // The vertex.instance_index is very important. Esepecially if you are using batching and gpu preprocessing
+    var world_from_local = mesh_functions::get_world_from_local(vertex.instance_index);
+    out.world_position = mesh_functions::mesh_position_local_to_world(world_from_local, vec4(vertex.position, 1.0));
+    out.clip_position = position_world_to_clip(out.world_position.xyz);
+
+    // We just use the raw vertex color
+    out.color = vertex.color.rgb;
+
+    return out;
+}
+
+@fragment
+fn fragment(in: VertexOutput) -> @location(0) vec4<f32> {
+    // output the color directly
+    return vec4(in.color, 1.0);
+}

examples/README.md

@@ -392,6 +392,7 @@ Example | Description
 [Material Prepass](../examples/shader/shader_prepass.rs) | A shader that uses the various textures generated by the prepass
 [Post Processing - Custom Render Pass](../examples/shader/custom_post_processing.rs) | A custom post processing effect, using a custom render pass that runs after the main pass
 [Shader Defs](../examples/shader/shader_defs.rs) | A shader that uses "shaders defs" (a bevy tool to selectively toggle parts of a shader)
+[Specialized Mesh Pipeline](../examples/shader/specialized_mesh_pipeline.rs) | Demonstrates how to write a specialized mesh pipeline
 [Texture Binding Array (Bindless Textures)](../examples/shader/texture_binding_array.rs) | A shader that shows how to bind and sample multiple textures as a binding array (a.k.a. bindless textures).
 
 ## State