Add headless mode (#3439)

# Objective

In this PR I added the ability to opt-out graphical backends. Closes #3155.

## Solution

I turned backends into `Option` ~~and removed panicking sub app API to force users handle the error (was suggested by `@cart`)~~.

Cargo.toml

@@ -249,6 +249,10 @@ path = "examples/app/return_after_run.rs"
 name = "thread_pool_resources"
 path = "examples/app/thread_pool_resources.rs"
 
+[[example]]
+name = "headless_defaults"
+path = "examples/app/headless_defaults.rs"
+
 [[example]]
 name = "without_winit"
 path = "examples/app/without_winit.rs"

crates/bevy_core_pipeline/src/lib.rs

@@ -95,7 +95,11 @@ impl Plugin for CorePipelinePlugin {
     fn build(&self, app: &mut App) {
         app.init_resource::<ClearColor>();
 
-        let render_app = app.sub_app_mut(RenderApp);
+        let render_app = match app.get_sub_app_mut(RenderApp) {
+            Ok(render_app) => render_app,
+            Err(_) => return,
+        };
+
         render_app
             .init_resource::<DrawFunctions<Transparent2d>>()
             .init_resource::<DrawFunctions<Opaque3d>>()

crates/bevy_pbr/src/lib.rs

@@ -138,7 +138,11 @@ impl Plugin for PbrPlugin {
                 },
             );
 
-        let render_app = app.sub_app_mut(RenderApp);
+        let render_app = match app.get_sub_app_mut(RenderApp) {
+            Ok(render_app) => render_app,
+            Err(_) => return,
+        };
+
         render_app
             .add_system_to_stage(
                 RenderStage::Extract,

crates/bevy_pbr/src/render/mesh.rs

@@ -53,11 +53,13 @@ impl Plugin for MeshRenderPlugin {
 
         app.add_plugin(UniformComponentPlugin::<MeshUniform>::default());
 
-        app.sub_app_mut(RenderApp)
-            .init_resource::<MeshPipeline>()
-            .add_system_to_stage(RenderStage::Extract, extract_meshes)
-            .add_system_to_stage(RenderStage::Queue, queue_mesh_bind_group)
-            .add_system_to_stage(RenderStage::Queue, queue_mesh_view_bind_groups);
+        if let Ok(render_app) = app.get_sub_app_mut(RenderApp) {
+            render_app
+                .init_resource::<MeshPipeline>()
+                .add_system_to_stage(RenderStage::Extract, extract_meshes)
+                .add_system_to_stage(RenderStage::Queue, queue_mesh_bind_group)
+                .add_system_to_stage(RenderStage::Queue, queue_mesh_view_bind_groups);
+        }
     }
 }
 

crates/bevy_pbr/src/wireframe.rs

@@ -31,13 +31,15 @@ impl Plugin for WireframePlugin {
 
         app.init_resource::<WireframeConfig>();
 
-        app.sub_app_mut(RenderApp)
-            .add_render_command::<Opaque3d, DrawWireframes>()
-            .init_resource::<WireframePipeline>()
-            .init_resource::<SpecializedPipelines<WireframePipeline>>()
-            .add_system_to_stage(RenderStage::Extract, extract_wireframes)
-            .add_system_to_stage(RenderStage::Extract, extract_wireframe_config)
-            .add_system_to_stage(RenderStage::Queue, queue_wireframes);
+        if let Ok(render_app) = app.get_sub_app_mut(RenderApp) {
+            render_app
+                .add_render_command::<Opaque3d, DrawWireframes>()
+                .init_resource::<WireframePipeline>()
+                .init_resource::<SpecializedPipelines<WireframePipeline>>()
+                .add_system_to_stage(RenderStage::Extract, extract_wireframes)
+                .add_system_to_stage(RenderStage::Extract, extract_wireframe_config)
+                .add_system_to_stage(RenderStage::Queue, queue_wireframes);
+        }
     }
 }
 

crates/bevy_render/src/camera/mod.rs

@@ -53,9 +53,11 @@ impl Plugin for CameraPlugin {
                 CoreStage::PostUpdate,
                 crate::camera::camera_system::<PerspectiveProjection>,
             );
-        app.sub_app_mut(RenderApp)
-            .init_resource::<ExtractedCameraNames>()
-            .add_system_to_stage(RenderStage::Extract, extract_cameras);
+        if let Ok(render_app) = app.get_sub_app_mut(RenderApp) {
+            render_app
+                .init_resource::<ExtractedCameraNames>()
+                .add_system_to_stage(RenderStage::Extract, extract_cameras);
+        }
     }
 }