Added visibility bitmask as an alternative SSAO method (#13454)

Early implementation. I still have to fix the documentation and consider
writing a small migration guide.

Questions left to answer:

* [x] should thickness be an overridable constant?
* [x] is there a better way to implement `Eq`/`Hash` for `SSAOMethod`?
* [x] do we want to keep the linear sampler for the depth texture?
* [x] is there a better way to separate the logic than preprocessor
macros?


![vbao](https://github.com/bevyengine/bevy/assets/4136413/2a8a0389-2add-4c2e-be37-e208e52dcd25)

## Migration guide

SSAO algorithm was changed from GTAO to VBAO (visibility bitmasks). A
new field, `constant_object_thickness`, was added to
`ScreenSpaceAmbientOcclusion`. `ScreenSpaceAmbientOcclusion` also lost
its `Eq` and `Hash` implementations.

---------

Co-authored-by: JMS55 <47158642+JMS55@users.noreply.github.com>

crates/bevy_pbr/src/deferred/deferred_lighting.wgsl

@@ -10,7 +10,7 @@
 
 #ifdef SCREEN_SPACE_AMBIENT_OCCLUSION
 #import bevy_pbr::mesh_view_bindings::screen_space_ambient_occlusion_texture
-#import bevy_pbr::gtao_utils::gtao_multibounce
+#import bevy_pbr::ssao_utils::ssao_multibounce
 #endif
 
 struct FullscreenVertexOutput {
@@ -64,7 +64,7 @@ fn fragment(in: FullscreenVertexOutput) -> @location(0) vec4<f32> {
 
 #ifdef SCREEN_SPACE_AMBIENT_OCCLUSION
  let ssao = textureLoad(screen_space_ambient_occlusion_texture, vec2<i32>(in.position.xy), 0i).r;
- let ssao_multibounce = gtao_multibounce(ssao, pbr_input.material.base_color.rgb);
+ let ssao_multibounce = ssao_multibounce(ssao, pbr_input.material.base_color.rgb);
  pbr_input.diffuse_occlusion = min(pbr_input.diffuse_occlusion, ssao_multibounce);
 
  // Neubelt and Pettineo 2013, "Crafting a Next-gen Material Pipeline for The Order: 1886"

crates/bevy_pbr/src/render/mesh_view_bindings.rs

@@ -44,7 +44,7 @@ use crate::{
  },
  prepass, EnvironmentMapUniformBuffer, FogMeta, GlobalClusterableObjectMeta,
  GpuClusterableObjects, GpuFog, GpuLights, LightMeta, LightProbesBuffer, LightProbesUniform,
- MeshPipeline, MeshPipelineKey, RenderViewLightProbes, ScreenSpaceAmbientOcclusionTextures,
+ MeshPipeline, MeshPipelineKey, RenderViewLightProbes, ScreenSpaceAmbientOcclusionResources,
  ScreenSpaceReflectionsBuffer, ScreenSpaceReflectionsUniform, ShadowSamplers,
  ViewClusterBindings, ViewShadowBindings, CLUSTERED_FORWARD_STORAGE_BUFFER_COUNT,
 };
@@ -462,7 +462,7 @@ pub fn prepare_mesh_view_bind_groups(
  &ViewShadowBindings,
  &ViewClusterBindings,
  &Msaa,
- Option<&ScreenSpaceAmbientOcclusionTextures>,
+ Option<&ScreenSpaceAmbientOcclusionResources>,
  Option<&ViewPrepassTextures>,
  Option<&ViewTransmissionTexture>,
  &Tonemapping,
@@ -507,7 +507,7 @@ pub fn prepare_mesh_view_bind_groups(
  shadow_bindings,
  cluster_bindings,
  msaa,
- ssao_textures,
+ ssao_resources,
  prepass_textures,
  transmission_texture,
  tonemapping,
@@ -519,7 +519,7 @@ pub fn prepare_mesh_view_bind_groups(
  .image_for_samplecount(1, TextureFormat::bevy_default())
  .texture_view
  .clone();
- let ssao_view = ssao_textures
+ let ssao_view = ssao_resources
  .map(|t| &t.screen_space_ambient_occlusion_texture.default_view)
  .unwrap_or(&fallback_ssao);
 

crates/bevy_pbr/src/render/pbr_fragment.wgsl

@@ -15,7 +15,7 @@
 
 #ifdef SCREEN_SPACE_AMBIENT_OCCLUSION
 #import bevy_pbr::mesh_view_bindings::screen_space_ambient_occlusion_texture
-#import bevy_pbr::gtao_utils::gtao_multibounce
+#import bevy_pbr::ssao_utils::ssao_multibounce
 #endif
 
 #ifdef MESHLET_MESH_MATERIAL_PASS
@@ -344,7 +344,7 @@ fn pbr_input_from_standard_material(
 #endif
 #ifdef SCREEN_SPACE_AMBIENT_OCCLUSION
  let ssao = textureLoad(screen_space_ambient_occlusion_texture, vec2<i32>(in.position.xy), 0i).r;
- let ssao_multibounce = gtao_multibounce(ssao, pbr_input.material.base_color.rgb);
+ let ssao_multibounce = ssao_multibounce(ssao, pbr_input.material.base_color.rgb);
  diffuse_occlusion = min(diffuse_occlusion, ssao_multibounce);
  // Use SSAO to estimate the specular occlusion.
  // Lagarde and Rousiers 2014, "Moving Frostbite to Physically Based Rendering"