Make vertex colors work without textures in bevy_sprite (#5685)

# Objective

This PR changes it possible to use vertex colors without a texture using the bevy_sprite ColorMaterial.

Fixes #5679 

## Solution

- Made multiplication of the output color independent of the COLOR_MATERIAL_FLAGS_TEXTURE_BIT bit
- Extended mesh2d_vertex_color_texture example to show off both vertex colors and tinting

Not sure if extending the existing example was the right call but it seems to be reasonable to me.

I couldn't find any tests for the shaders and I think adding shader testing would be beyond the scope of this PR. So no tests in this PR. 😬 

Co-authored-by: Jonas Wagner <jonas@29a.ch>

crates/bevy_sprite/src/mesh2d/color_material.wgsl

@@ -26,12 +26,11 @@ struct FragmentInput {
 @fragment
 fn fragment(in: FragmentInput) -> @location(0) vec4<f32> {
     var output_color: vec4<f32> = material.color;
-    if ((material.flags & COLOR_MATERIAL_FLAGS_TEXTURE_BIT) != 0u) {
 #ifdef VERTEX_COLORS
-        output_color = output_color * textureSample(texture, texture_sampler, in.uv) * in.color;
-#else
-        output_color = output_color * textureSample(texture, texture_sampler, in.uv);
+    output_color = output_color * in.color;
 #endif
+    if ((material.flags & COLOR_MATERIAL_FLAGS_TEXTURE_BIT) != 0u) {
+        output_color = output_color * textureSample(texture, texture_sampler, in.uv);
     }
     return output_color;
 }

examples/2d/mesh2d_vertex_color_texture.rs

@@ -1,7 +1,10 @@
 //! Shows how to render a polygonal [`Mesh`], generated from a [`Quad`] primitive, in a 2D scene.
 //! Adds a texture and colored vertices, giving per-vertex tinting.
 
-use bevy::{prelude::*, sprite::MaterialMesh2dBundle};
+use bevy::{
+    prelude::*,
+    sprite::{MaterialMesh2dBundle, Mesh2dHandle},
+};
 
 fn main() {
     App::new()
@@ -29,11 +32,26 @@ fn setup(
     ];
     // Insert the vertex colors as an attribute
     mesh.insert_attribute(Mesh::ATTRIBUTE_COLOR, vertex_colors);
-    // Spawn
+
+    let mesh_handle: Mesh2dHandle = meshes.add(mesh).into();
+
+    // Spawn camera
     commands.spawn_bundle(Camera2dBundle::default());
+
+    // Spawn the quad with vertex colors
     commands.spawn_bundle(MaterialMesh2dBundle {
-        mesh: meshes.add(mesh).into(),
-        transform: Transform::default().with_scale(Vec3::splat(128.)),
+        mesh: mesh_handle.clone(),
+        transform: Transform::from_translation(Vec3::new(-96., 0., 0.))
+            .with_scale(Vec3::splat(128.)),
+        material: materials.add(ColorMaterial::default()),
+        ..default()
+    });
+
+    // Spawning the quad with vertex colors and a texture results in tinting
+    commands.spawn_bundle(MaterialMesh2dBundle {
+        mesh: mesh_handle,
+        transform: Transform::from_translation(Vec3::new(96., 0., 0.))
+            .with_scale(Vec3::splat(128.)),
         material: materials.add(ColorMaterial::from(texture_handle)),
         ..default()
     });