Move AlphaMode into bevy_render (#12012)

# Objective

- Closes #11985

## Solution

- alpha.rs has been moved from bevy_pbr into bevy_render; bevy_pbr and
bevy_gltf now access `AlphaMode` through bevy_render.

---

## Migration Guide

In the present implementation, external consumers of `AlphaMode` will
have to access it through bevy_render rather than through bevy_pbr,
changing their import from `bevy_pbr::AlphaMode` to
`bevy_render::alpha::AlphaMode` (or the corresponding glob import from
`bevy_pbr::prelude::*` to `bevy_render::prelude::*`).

## Uncertainties

Some remaining things from this that I am uncertain about:
- Here, the `app.register_type<AlphaMode>()` call has been moved from
`PbrPlugin` to `RenderPlugin`; I'm not sure if this is quite right, and
I was unable to find any direct relationship between `PbrPlugin` and
`RenderPlugin`.
- `AlphaMode` was placed in the prelude of bevy_render. I'm not certain
that this is actually appropriate.
- bevy_pbr does not re-export `AlphaMode`, which makes this a breaking
change for external consumers.

Any of these things could be easily changed; I'm just not confident that
I necessarily adopted the right approach in these (known) ways since
this codebase and ecosystem is quite new to me.

crates/bevy_gltf/src/loader.rs

@@ -11,10 +11,11 @@ use bevy_hierarchy::{BuildWorldChildren, WorldChildBuilder};
 use bevy_log::{error, info_span, warn};
 use bevy_math::{Affine2, Mat4, Vec3};
 use bevy_pbr::{
-    AlphaMode, DirectionalLight, DirectionalLightBundle, PbrBundle, PointLight, PointLightBundle,
-    SpotLight, SpotLightBundle, StandardMaterial, MAX_JOINTS,
+    DirectionalLight, DirectionalLightBundle, PbrBundle, PointLight, PointLightBundle, SpotLight,
+    SpotLightBundle, StandardMaterial, MAX_JOINTS,
 };
 use bevy_render::{
+    alpha::AlphaMode,
     camera::{Camera, OrthographicProjection, PerspectiveProjection, Projection, ScalingMode},
     color::Color,
     mesh::{

crates/bevy_pbr/src/lib.rs

@@ -3,7 +3,6 @@
 
 pub mod wireframe;
 
-mod alpha;
 mod bundle;
 pub mod deferred;
 mod extended_material;
@@ -18,7 +17,6 @@ mod prepass;
 mod render;
 mod ssao;
 
-pub use alpha::*;
 pub use bundle::*;
 pub use extended_material::*;
 pub use fog::*;
@@ -35,7 +33,6 @@ pub use ssao::*;
 pub mod prelude {
     #[doc(hidden)]
     pub use crate::{
-        alpha::AlphaMode,
         bundle::{
             DirectionalLightBundle, MaterialMeshBundle, PbrBundle, PointLightBundle,
             SpotLightBundle,
@@ -72,6 +69,7 @@ use bevy_asset::{load_internal_asset, AssetApp, Assets, Handle};
 use bevy_core_pipeline::core_3d::graph::{Core3d, Node3d};
 use bevy_ecs::prelude::*;
 use bevy_render::{
+    alpha::AlphaMode,
     camera::{CameraUpdateSystem, Projection},
     extract_component::ExtractComponentPlugin,
     extract_resource::ExtractResourcePlugin,
@@ -235,7 +233,6 @@ impl Plugin for PbrPlugin {
         );
 
         app.register_asset_reflect::<StandardMaterial>()
-            .register_type::<AlphaMode>()
             .register_type::<AmbientLight>()
             .register_type::<Cascade>()
             .register_type::<CascadeShadowConfig>()

crates/bevy_render/src/lib.rs

@@ -6,6 +6,7 @@ compile_error!("bevy_render cannot compile for a 16-bit platform.");
 
 extern crate core;
 
+pub mod alpha;
 pub mod batching;
 pub mod camera;
 pub mod color;
@@ -32,6 +33,7 @@ pub mod view;
 pub mod prelude {
     #[doc(hidden)]
     pub use crate::{
+        alpha::AlphaMode,
         camera::{
             Camera, ClearColor, ClearColorConfig, OrthographicProjection, PerspectiveProjection,
             Projection,
@@ -327,7 +329,8 @@ impl Plugin for RenderPlugin {
             MorphPlugin,
         ));
 
-        app.register_type::<color::Color>()
+        app.register_type::<alpha::AlphaMode>()
+            .register_type::<color::Color>()
             .register_type::<primitives::Aabb>()
             .register_type::<primitives::CascadesFrusta>()
             .register_type::<primitives::CubemapFrusta>()