Mixamo animation support (#37)

This PR adds support for Mixamo FBX imports (via backend conversion to
glTF/glB) and some miscellaneous improvements to the animation systems.

### User-facing improvements
* Retargeting math has been fixed to eliminate jittering issues
* Retargeter can now handle animated bone translations when present
  (generally this is only used for root motion)

### Housekeeping / Code improvements

* Animation-related components and system logic have been moved to a
  dedicated `anim` module and `AnimPlugin`
* Retargeting stuff has been moved from the `bvh` module to
  `anim::retargeting`
* Component-based solution for marking an entity as an animation
  retargeting source makes it easy to extend the retargeting functionality
  to new types of source animations

### Limitations

Currently the code assumes that there will be at most one animation
source at a time. That single animation (if present) will be retargeted
to all other skeletons in the scene. A component-based solution for
directing a specific target skeleton to use a specific source animation
is possible, but will require some additional consideration around the
implementation design for the read-only "viewer" modes.

### Test animations:

* Hip-Hop Dancing (Mixamo): ?mode=viewer&mixamo=mocap/hip-hop-dancing.gltf
* Shuffle (MocapNET): ?mode=viewer&bvh=mocap/shuffle.bvh
* Scott (MocapNET): ?mode=viewer&bvh=mocap/scott.bvh
* Scott 1 (MocapNET): ?mode=viewer&bvh=mocap/scott1.bvh

studio/src/lib.rs

@@ -2,20 +2,22 @@
 
 use bevy::{app::PluginGroupBuilder, prelude::*};
 use bevy_mod_picking::DefaultPickingPlugins;
-
 use bevy_web_asset::WebAssetPlugin;
-use bvh::BvhPlugin;
+
 #[cfg(feature = "wasm")]
 use wasm_bindgen::prelude::*;
 
+mod anim;
 mod bvh;
 mod camera_controller;
 mod debug;
 mod design_system;
+mod hierarchy;
 mod input;
 mod inspector;
 mod interaction;
 mod math;
+mod mixamo;
 mod persistance;
 mod scene;
 mod skybox;
@@ -24,12 +26,16 @@ pub mod viewport;
 mod world_grid;
 
 pub use crate::{
+    anim::AnimPlugin,
+    bvh::BvhPlugin,
     camera_controller::CameraControllerPlugin,
     debug::DebugPlugin,
     input::StudioInputPlugin,
     inspector::InspectorPlugin,
     interaction::InteractionPlugin,
     math::InterpolationPlugin,
+    mixamo::{MixamoImportEvent, MixamoPlugin},
+    persistance::PersistancePlugin,
     scene::{LoadSceneEvent, ScenePlugin},
     skybox::{SkyboxId, SkyboxPlugin},
     ui::UiLayerPlugin,
@@ -69,8 +75,8 @@ impl PluginGroup for CommonPlugins {
             .add(StudioInputPlugin)
             .add(UiLayerPlugin)
             .add(WorldGridPlugin)
-            .add(persistance::PersistancePlugin)
-            .add(BvhPlugin)
+            .add(PersistancePlugin)
+            .add(AnimPlugin)
     }
 }
 
@@ -149,16 +155,44 @@ pub fn start_bvh_viewer(
 
     let bvh_path = bvh_path.to_string();
 
+    app.add_plugins(BvhPlugin);
     app.add_systems(
         Startup,
         move |mut cmd: Commands, r_assets: Res<AssetServer>| {
-            cmd.insert_resource(scene::TestBvh(r_assets.load(bvh_path.clone())));
+            cmd.insert_resource(bvh::TestBvh(r_assets.load(bvh_path.clone())));
         },
     );
 
     app.run();
 }
 
+#[cfg_attr(feature = "wasm", wasm_bindgen(js_name = startMixamoViewer))]
+pub fn start_mixamo_viewer(
+    mixamo_path: &str,
+    skybox_id: &str,
+    canvas_selector: &str,
+    canvas_alt: &str,
+    viewport_size: ViewportSize,
+) {
+    let mut app = start_internal(
+        "Mannequin.gltf",
+        skybox_id,
+        canvas_selector,
+        canvas_alt,
+        viewport_size,
+        StudioMode::Viewer,
+    );
+
+    let mixamo_path = mixamo_path.to_string();
+
+    app.add_plugins(MixamoPlugin);
+    app.add_systems(Startup, move |mut ew: EventWriter<MixamoImportEvent>| {
+        ew.send(MixamoImportEvent(mixamo_path.clone()));
+    });
+
+    app.run();
+}
+
 #[cfg_attr(feature = "wasm", wasm_bindgen)]
 pub fn start(
     object_id: &str,