Implement bindless lightmaps. (#16653)

This commit allows Bevy to bind 16 lightmaps at a time, if the current
platform supports bindless textures. Naturally, if bindless textures
aren't supported, Bevy falls back to binding only a single lightmap at a
time. As lightmaps are usually heavily atlased, I doubt many scenes will
use more than 16 lightmap textures.

This has little performance impact now, but it's desirable for us to
reap the benefits of multidraw and bindless textures on scenes that use
lightmaps. Otherwise, we might have to break batches in order to switch
those lightmaps.

Additionally, this PR slightly reduces the cost of binning because it
makes the lightmap index in `Opaque3dBinKey` 32 bits instead of an
`AssetId`.

## Migration Guide

* The `Opaque3dBinKey::lightmap_image` field is now
`Opaque3dBinKey::lightmap_slab`, which is a lightweight identifier for
an entire binding array of lightmaps.

assets/shaders/bindless_material.wgsl

@@ -18,7 +18,7 @@ struct Color {
 @fragment
 fn fragment(in: VertexOutput) -> @location(0) vec4<f32> {
 #ifdef BINDLESS
-    let slot = mesh[in.instance_index].material_bind_group_slot;
+    let slot = mesh[in.instance_index].material_and_lightmap_bind_group_slot & 0xffffu;
     let base_color = material_color[slot].base_color;
 #else   // BINDLESS
     let base_color = material_color.base_color;

crates/bevy_core_pipeline/src/core_3d/mod.rs

@@ -76,10 +76,10 @@ pub use main_opaque_pass_3d_node::*;
 pub use main_transparent_pass_3d_node::*;
 
 use bevy_app::{App, Plugin, PostUpdate};
-use bevy_asset::{AssetId, UntypedAssetId};
+use bevy_asset::UntypedAssetId;
 use bevy_color::LinearRgba;
 use bevy_ecs::{entity::EntityHashSet, prelude::*};
-use bevy_image::{BevyDefault, Image};
+use bevy_image::BevyDefault;
 use bevy_math::FloatOrd;
 use bevy_render::{
     camera::{Camera, ExtractedCamera},
@@ -102,6 +102,7 @@ use bevy_render::{
     Extract, ExtractSchedule, Render, RenderApp, RenderSet,
 };
 use bevy_utils::{tracing::warn, HashMap};
+use nonmax::NonMaxU32;
 
 use crate::{
     core_3d::main_transmissive_pass_3d_node::MainTransmissivePass3dNode,
@@ -258,8 +259,9 @@ pub struct Opaque3dBatchSetKey {
     /// For non-mesh items, you can safely fill this with `None`.
     pub index_slab: Option<SlabId>,
 
-    /// The lightmap, if present.
-    pub lightmap_image: Option<AssetId<Image>>,
+    /// Index of the slab that the lightmap resides in, if a lightmap is
+    /// present.
+    pub lightmap_slab: Option<NonMaxU32>,
 }
 
 /// Data that must be identical in order to *batch* phase items together.

crates/bevy_pbr/src/lightmap/lightmap.wgsl

@@ -2,8 +2,13 @@
 
 #import bevy_pbr::mesh_bindings::mesh
 
+#ifdef MULTIPLE_LIGHTMAPS_IN_ARRAY
+@group(1) @binding(4) var lightmaps_textures: binding_array<texture_2d<f32>>;
+@group(1) @binding(5) var lightmaps_samplers: binding_array<sampler>;
+#else   // MULTIPLE_LIGHTMAPS_IN_ARRAY
 @group(1) @binding(4) var lightmaps_texture: texture_2d<f32>;
 @group(1) @binding(5) var lightmaps_sampler: sampler;
+#endif  // MULTIPLE_LIGHTMAPS_IN_ARRAY
 
 // Samples the lightmap, if any, and returns indirect illumination from it.
 fn lightmap(uv: vec2<f32>, exposure: f32, instance_index: u32) -> vec3<f32> {
@@ -21,9 +26,20 @@ fn lightmap(uv: vec2<f32>, exposure: f32, instance_index: u32) -> vec3<f32> {
     // control flow uniformity problems.
     //
     // TODO(pcwalton): Consider bicubic filtering.
+#ifdef MULTIPLE_LIGHTMAPS_IN_ARRAY
+    let lightmap_slot = mesh[instance_index].material_and_lightmap_bind_group_slot >> 16u;
+    return textureSampleLevel(
+        lightmaps_textures[lightmap_slot],
+        lightmaps_samplers[lightmap_slot],
+        lightmap_uv,
+        0.0
+    ).rgb * exposure;
+#else   // MULTIPLE_LIGHTMAPS_IN_ARRAY
     return textureSampleLevel(
         lightmaps_texture,
         lightmaps_sampler,
         lightmap_uv,
-        0.0).rgb * exposure;
+        0.0
+    ).rgb * exposure;
+#endif  // MULTIPLE_LIGHTMAPS_IN_ARRAY
 }