Refactor Studio entry-points (#54)

This PR heavily refactors the entry-point functions in
`studio/src/lib.rs` to simplify and consolidate. Additionally, the
frontend application should now be configurable as expected with any
valid combination of the following URL parameters:

* `objectId` : A pre-bundled library object to load (e.g., "couch.gltf")
* `bvh` : Any resolvable path to a BVH animation with a MocapNET
skeleton (e.g., "mocap/shuffle.bvh")
* `mixamo` : Any resolvable path to a glTF animation with the Mixamo
skeleton (e.g., "mocap/hip-hop-dancing.gltf")
* `sceneImport` : Any resolvable path to an arbitrary glTF scene
(untested, but should theoretically work)
* `scene` : Any resolvable path to a previously saved Storyteller Studio
scene in `*.scn.ron` format
* `skybox` : The ID of a pre-bundled skybox asset. Valid options are:
  - "gum_trees_4k"
  - "kloofenfal_28d_misty_4k"
  - "meadow_4k"
  - "promenade_de_vidy_4k"
  - "scythian_tombs_4k"
  - "test_scene" (this is the default if not specified)
* `mode` : One of "studio" or "viewer" (the default if not specified)

"Validity" rules are as follows:

* `objectId`, `bvh`, `mixamo`, `sceneImport` and `scene` are mutually
exclusive
* `skybox` is not supported for `scene` (the skybox should be saved and
loaded as part of the scene description, though this is not currently
working)
* Both `mode` options are valid for all other parameters

studio/src/scene.rs

@@ -26,10 +26,10 @@ use crate::{
     camera_controller::{CameraController, CameraControllerBundle},
     change_tracking::{ChangeTracker, ChangeTrackingPlugin},
     debug::DebugPoint,
-    is_headless, is_not_headless,
+    headless::{is_headless, is_not_headless, HeadlessSettings},
     math::{Combinable, Extrema},
     screenshot::ScreenCapture,
-    HeadlessSettings, MainCamera,
+    MainCamera,
 };
 
 pub struct ScenePlugin;
@@ -46,6 +46,7 @@ impl Plugin for ScenePlugin {
             .add_state::<SceneState>()
             .add_event::<LoadLibraryObjectEvent>()
             .add_event::<LoadSceneEvent>()
+            .add_event::<ImportSceneEvent>()
             .add_systems(
                 Startup,
                 (
@@ -55,7 +56,8 @@ impl Plugin for ScenePlugin {
             )
             .add_systems(
                 Update,
-                (init_object_loading, init_scene_loading).in_set(SceneSpawnerSystem),
+                (init_object_loading, init_scene_import, init_scene_loading)
+                    .in_set(SceneSpawnerSystem),
             )
             .add_systems(
                 OnEnter(SceneState::Active),
@@ -208,8 +210,7 @@ fn auto_add_handle<T: Asset>(
 #[derive(Event, Deref)]
 pub struct LoadLibraryObjectEvent(pub String);
 
-/// Request the application to load a previously saved `*.scn.ron` dynamic scene
-/// file.
+/// Request the application to load a previously saved Storyteller Studio scene.
 ///
 /// Expects a full, resolvable path to the file (e.g. `scenes/my-scene.scn.ron`
 /// for a local file saved under `studio/assets/scenes`, or
@@ -218,6 +219,11 @@ pub struct LoadLibraryObjectEvent(pub String);
 #[derive(Event, Deref)]
 pub struct LoadSceneEvent(pub String);
 
+/// Request the application to import and process a glTF scene from an arbitrary
+/// asset source (e.g., an http path or a Storyteller.ai media token).
+#[derive(Event, Deref)]
+pub struct ImportSceneEvent(pub String);
+
 // TODOL This could be converted into an enum that indicates what "kind" of
 //       element this is (e.g. StaticMesh, SkinnedMesh, Bone, Light, Camera).
 //       We don't have an immediate need for that just yet, but likely will
@@ -329,6 +335,21 @@ fn init_object_loading(mut cmd: Commands, mut er: EventReader<LoadLibraryObjectE
     }
 }
 
+fn init_scene_import(mut cmd: Commands, mut er: EventReader<ImportSceneEvent>) {
+    if let Some(path) = er.read().last() {
+        info!("Importing asset: {}", path.0);
+        cmd.spawn((
+            SpatialBundle::default(),
+            space_editor::prelude::GltfPrefab {
+                path: path.0.clone(),
+                scene: "Scene0".to_string(),
+            },
+            PrefabMarker,
+            SceneRoot,
+        ));
+    }
+}
+
 fn init_scene_loading(
     mut er_load_scene: EventReader<LoadSceneEvent>,
     mut ew_editor: EventWriter<EditorEvent>,