use alpha mask even when unlit (#6047)

# Objective

- Alpha mask was previously ignored when using an unlit material. 
- Fixes https://github.com/bevyengine/bevy/issues/4479

## Solution

- Extract the alpha discard to a separate function and use it when unlit is true

## Notes
I tried calling `alpha_discard()` before the `if` in pbr.wgsl, but I had errors related to having a `discard` at the beginning before doing the texture sampling. I'm not sure if there's a way to fix that instead of having the function being called in 2 places.

crates/bevy_pbr/src/render/pbr.wgsl

@@ -89,6 +89,8 @@ fn fragment(in: FragmentInput) -> @location(0) vec4<f32> {
         pbr_input.V = calculate_view(in.world_position, pbr_input.is_orthographic);
 
         output_color = tone_mapping(pbr(pbr_input));
+    } else {
+        output_color = alpha_discard(material, output_color);
     }
 
     return output_color;

crates/bevy_pbr/src/render/pbr_functions.wgsl

@@ -1,5 +1,23 @@
 #define_import_path bevy_pbr::pbr_functions
 
+fn alpha_discard(material: StandardMaterial, output_color: vec4<f32>) -> vec4<f32>{
+    var color = output_color;
+    if ((material.flags & STANDARD_MATERIAL_FLAGS_ALPHA_MODE_OPAQUE) != 0u) {
+        // NOTE: If rendering as opaque, alpha should be ignored so set to 1.0
+        color.a = 1.0;
+    } else if ((material.flags & STANDARD_MATERIAL_FLAGS_ALPHA_MODE_MASK) != 0u) {
+        if (color.a >= material.alpha_cutoff) {
+            // NOTE: If rendering as masked alpha and >= the cutoff, render as fully opaque
+            color.a = 1.0;
+        } else {
+            // NOTE: output_color.a < in.material.alpha_cutoff should not is not rendered
+            // NOTE: This and any other discards mean that early-z testing cannot be done!
+            discard;
+        }
+    }
+    return color;
+}
+
 // NOTE: This ensures that the world_normal is normalized and if
 // vertex tangents and normal maps then normal mapping may be applied.
 fn prepare_normal(
@@ -142,19 +160,7 @@ fn pbr(
 
     let occlusion = in.occlusion;
 
-    if ((in.material.flags & STANDARD_MATERIAL_FLAGS_ALPHA_MODE_OPAQUE) != 0u) {
-        // NOTE: If rendering as opaque, alpha should be ignored so set to 1.0
-        output_color.a = 1.0;
-    } else if ((in.material.flags & STANDARD_MATERIAL_FLAGS_ALPHA_MODE_MASK) != 0u) {
-        if (output_color.a >= in.material.alpha_cutoff) {
-            // NOTE: If rendering as masked alpha and >= the cutoff, render as fully opaque
-            output_color.a = 1.0;
-        } else {
-            // NOTE: output_color.a < in.material.alpha_cutoff should not is not rendered
-            // NOTE: This and any other discards mean that early-z testing cannot be done!
-            discard;
-        }
-    }
+    output_color = alpha_discard(in.material, output_color);
 
     // Neubelt and Pettineo 2013, "Crafting a Next-gen Material Pipeline for The Order: 1886"
     let NdotV = max(dot(in.N, in.V), 0.0001);
@@ -247,3 +253,4 @@ fn tone_mapping(in: vec4<f32>) -> vec4<f32> {
     // Not needed with sRGB buffer
     // output_color.rgb = pow(output_color.rgb, vec3(1.0 / 2.2));
 }
+

examples/3d/transparency_3d.rs

@@ -44,6 +44,21 @@ fn setup(
         transform: Transform::from_xyz(1.0, 0.5, -1.5),
         ..default()
     });
+    // transparent unlit sphere, uses `alpha_mode: Mask(f32)`
+    commands.spawn(PbrBundle {
+        mesh: meshes.add(Mesh::from(shape::Icosphere {
+            radius: 0.5,
+            subdivisions: 3,
+        })),
+        material: materials.add(StandardMaterial {
+            base_color: Color::rgba(0.2, 0.7, 0.1, 0.0),
+            alpha_mode: AlphaMode::Mask(0.5),
+            unlit: true,
+            ..default()
+        }),
+        transform: Transform::from_xyz(-1.0, 0.5, -1.5),
+        ..default()
+    });
     // transparent cube, uses `alpha_mode: Blend`
     commands.spawn(PbrBundle {
         mesh: meshes.add(Mesh::from(shape::Cube { size: 1.0 })),