Fix lightmaps break when deferred rendering is enabled

crates/bevy_pbr/src/deferred/pbr_deferred_functions.wgsl

@@ -53,6 +53,22 @@ fn deferred_gbuffer_from_pbr_input(in: PbrInput) -> vec4<u32> {
     } else {
         base_color_srgb = pow(in.material.base_color.rgb, vec3(1.0 / 2.2));
     }
+
+    // Utilize the emissive channel to transmit the lightmap data. To ensure
+    // it matches the output in forward shading, pre-multiply it with the 
+    // calculated diffuse color.
+    let base_color = in.material.base_color.rgb;
+    let metallic = in.material.metallic;
+    let specular_transmission = in.material.specular_transmission;
+    let diffuse_transmission = in.material.diffuse_transmission;
+    let diffuse_color = pbr_functions::calculate_diffuse_color(
+        base_color,
+        metallic,
+        specular_transmission,
+        diffuse_transmission
+    );
+    emissive += in.lightmap_light * diffuse_color * view.exposure;
+
     let deferred = vec4(
         deferred_types::pack_unorm4x8_(vec4(base_color_srgb, in.material.perceptual_roughness)),
         rgb9e5::vec3_to_rgb9e5_(emissive),

crates/bevy_pbr/src/prepass/mod.rs

@@ -425,6 +425,10 @@ where
             shader_defs.push("DEFERRED_PREPASS".into());
         }
 
+        if key.mesh_key.contains(MeshPipelineKey::LIGHTMAPPED) {
+            shader_defs.push("LIGHTMAP".into());
+        }
+
         if layout.0.contains(Mesh::ATTRIBUTE_COLOR) {
             shader_defs.push("VERTEX_COLORS".into());
             vertex_attributes.push(Mesh::ATTRIBUTE_COLOR.at_shader_location(7));