Make UIPlugin not load rendering-dependent systems and resources if it's disabled

Author: kirusfg

crates/bevy_ui/src/lib.rs

@@ -71,8 +71,10 @@ impl Plugin for UiPlugin {
             .register_type::<UiImage>()
             .register_type::<Val>()
             .register_type::<widget::Button>()
-            .register_type::<widget::ImageMode>()
-            .add_system_to_stage(
+            .register_type::<widget::ImageMode>();
+
+        if cfg!(feature = "render") {
+            app.add_system_to_stage(
                 CoreStage::PreUpdate,
                 ui_focus_system.label(UiSystem::Focus).after(InputSystem),
             )
@@ -102,6 +104,7 @@ impl Plugin for UiPlugin {
                 update_clipping_system.after(TransformSystem::TransformPropagate),
             );
 
-        crate::render::build_ui_render(app);
+            crate::render::build_ui_render(app);
+        }
     }
 }