Use the prepass normal texture in main pass when possible (#8231)

# Objective

- We support enabling a normal prepass, but the main pass never actually
uses it and recomputes the normals in the main pass. This isn't ideal
since it's doing redundant work.

## Solution

- Use the normal texture from the prepass in the main pass

## Notes

~~I used `NORMAL_PREPASS_ENABLED` as a shader_def because
`NORMAL_PREPASS` is currently used to signify that it is running in the
prepass while this shader_def need to indicate the prepass is done and
the normal prepass was ran before. I'm not sure if there's a better way
to name this.~~

crates/bevy_pbr/src/material.rs

@@ -7,6 +7,7 @@ use bevy_app::{App, Plugin};
 use bevy_asset::{AddAsset, AssetEvent, AssetServer, Assets, Handle};
 use bevy_core_pipeline::{
     core_3d::{AlphaMask3d, Opaque3d, Transparent3d},
+    prepass::NormalPrepass,
     tonemapping::{DebandDither, Tonemapping},
 };
 use bevy_derive::{Deref, DerefMut};
@@ -380,6 +381,7 @@ pub fn queue_material_meshes<M: Material>(
         Option<&Tonemapping>,
         Option<&DebandDither>,
         Option<&EnvironmentMapLight>,
+        Option<&NormalPrepass>,
         &mut RenderPhase<Opaque3d>,
         &mut RenderPhase<AlphaMask3d>,
         &mut RenderPhase<Transparent3d>,
@@ -393,6 +395,7 @@ pub fn queue_material_meshes<M: Material>(
         tonemapping,
         dither,
         environment_map,
+        normal_prepass,
         mut opaque_phase,
         mut alpha_mask_phase,
         mut transparent_phase,
@@ -405,6 +408,10 @@ pub fn queue_material_meshes<M: Material>(
         let mut view_key = MeshPipelineKey::from_msaa_samples(msaa.samples())
             | MeshPipelineKey::from_hdr(view.hdr);
 
+        if normal_prepass.is_some() {
+            view_key |= MeshPipelineKey::NORMAL_PREPASS;
+        }
+
         let environment_map_loaded = match environment_map {
             Some(environment_map) => environment_map.is_loaded(&images),
             None => false,

crates/bevy_pbr/src/render/mesh.rs

@@ -665,6 +665,10 @@ impl SpecializedMeshPipeline for MeshPipeline {
         let mut shader_defs = Vec::new();
         let mut vertex_attributes = Vec::new();
 
+        if key.contains(MeshPipelineKey::NORMAL_PREPASS) {
+            shader_defs.push("LOAD_PREPASS_NORMALS".into());
+        }
+
         if layout.contains(Mesh::ATTRIBUTE_POSITION) {
             shader_defs.push("VERTEX_POSITIONS".into());
             vertex_attributes.push(Mesh::ATTRIBUTE_POSITION.at_shader_location(0));

crates/bevy_pbr/src/render/pbr.wgsl

@@ -10,6 +10,8 @@
 #import bevy_pbr::fog
 #import bevy_pbr::pbr_functions
 
+#import bevy_pbr::prepass_utils
+
 struct FragmentInput {
     @builtin(front_facing) is_front: bool,
     @builtin(position) frag_coord: vec4<f32>,
@@ -69,11 +71,16 @@ fn fragment(in: FragmentInput) -> @location(0) vec4<f32> {
 #endif
         pbr_input.frag_coord = in.frag_coord;
         pbr_input.world_position = in.world_position;
+
+#ifdef LOAD_PREPASS_NORMALS
+        pbr_input.world_normal = prepass_normal(in.frag_coord, 0u);
+#else // LOAD_PREPASS_NORMALS
         pbr_input.world_normal = prepare_world_normal(
             in.world_normal,
             (material.flags & STANDARD_MATERIAL_FLAGS_DOUBLE_SIDED_BIT) != 0u,
             in.is_front,
         );
+#endif // LOAD_PREPASS_NORMALS
 
         pbr_input.is_orthographic = view.projection[3].w == 1.0;