Remove dependency on the mesh struct in the pbr function (#7597)

# Objective

Currently, it is quite awkward to use the `pbr` function in a custom shader without binding a mesh bind group.
This is because the `pbr` function depends on the `MESH_FLAGS_SHADOW_RECEIVER_BIT` flag.

## Solution

I have removed this dependency by adding the flag as a parameter to the `PbrInput` struct.

I am not sure if this is the ideal solution since the mesh flag indicates both `MESH_FLAGS_SIGN_DETERMINANT_MODEL_3X3_BIT` and `MESH_FLAGS_SHADOW_RECEIVER_BIT`.
The former seems to be unrelated to PBR. Maybe the flag should be split.

crates/bevy_pbr/src/render/pbr.wgsl

@@ -92,6 +92,8 @@ fn fragment(in: FragmentInput) -> @location(0) vec4<f32> {
         pbr_input.V = calculate_view(in.world_position, pbr_input.is_orthographic);
         pbr_input.occlusion = occlusion;
 
+        pbr_input.flags = mesh.flags;
+
         output_color = pbr(pbr_input);
     } else {
         output_color = alpha_discard(material, output_color);

crates/bevy_pbr/src/render/pbr_functions.wgsl

@@ -134,6 +134,7 @@ struct PbrInput {
     // view world position
     V: vec3<f32>,
     is_orthographic: bool,
+    flags: u32,
 };
 
 // Creates a PbrInput with default values
@@ -152,6 +153,8 @@ fn pbr_input_new() -> PbrInput {
     pbr_input.N = vec3<f32>(0.0, 0.0, 1.0);
     pbr_input.V = vec3<f32>(1.0, 0.0, 0.0);
 
+    pbr_input.flags = 0u;
+
     return pbr_input;
 }
 
@@ -203,7 +206,7 @@ fn pbr(
     for (var i: u32 = offset_and_counts[0]; i < offset_and_counts[0] + offset_and_counts[1]; i = i + 1u) {
         let light_id = get_light_id(i);
         var shadow: f32 = 1.0;
-        if ((mesh.flags & MESH_FLAGS_SHADOW_RECEIVER_BIT) != 0u
+        if ((in.flags & MESH_FLAGS_SHADOW_RECEIVER_BIT) != 0u
                 && (point_lights.data[light_id].flags & POINT_LIGHT_FLAGS_SHADOWS_ENABLED_BIT) != 0u) {
             shadow = fetch_point_shadow(light_id, in.world_position, in.world_normal);
         }
@@ -215,7 +218,7 @@ fn pbr(
     for (var i: u32 = offset_and_counts[0] + offset_and_counts[1]; i < offset_and_counts[0] + offset_and_counts[1] + offset_and_counts[2]; i = i + 1u) {
         let light_id = get_light_id(i);
         var shadow: f32 = 1.0;
-        if ((mesh.flags & MESH_FLAGS_SHADOW_RECEIVER_BIT) != 0u
+        if ((in.flags & MESH_FLAGS_SHADOW_RECEIVER_BIT) != 0u
                 && (point_lights.data[light_id].flags & POINT_LIGHT_FLAGS_SHADOWS_ENABLED_BIT) != 0u) {
             shadow = fetch_spot_shadow(light_id, in.world_position, in.world_normal);
         }
@@ -227,7 +230,7 @@ fn pbr(
     let n_directional_lights = lights.n_directional_lights;
     for (var i: u32 = 0u; i < n_directional_lights; i = i + 1u) {
         var shadow: f32 = 1.0;
-        if ((mesh.flags & MESH_FLAGS_SHADOW_RECEIVER_BIT) != 0u
+        if ((in.flags & MESH_FLAGS_SHADOW_RECEIVER_BIT) != 0u
                 && (lights.directional_lights[i].flags & DIRECTIONAL_LIGHT_FLAGS_SHADOWS_ENABLED_BIT) != 0u) {
             shadow = fetch_directional_shadow(i, in.world_position, in.world_normal, view_z);
         }