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-frontend/src/app/app.element.ts

@@ -21,6 +21,7 @@ import { Ref, createRef, ref } from "lit-html/directives/ref.js";
 import "@storyteller/studio-web/canvas-provider";
 import "@storyteller/studio-web/viewer";
 import "@storyteller/studio-web/bvh-viewer";
+import "@storyteller/studio-web/mixamo-viewer";
 import "./button.element";
 import "./help.element";
 import "./icon.element";
@@ -46,6 +47,7 @@ export class AppElement extends LitElement {
 	@state() selection?: string | null;
 
 	@state() bvhPath?: string;
+	@state() mixamoPath?: string;
 	@state() mode?: StudioMode;
 
 	#sceneObjects: studio.SceneObject[] = [];
@@ -67,6 +69,9 @@ export class AppElement extends LitElement {
 		if (params.has("bvh"))
 			this.bvhPath = params.get("bvh")!;
 
+		if (params.has("mixamo"))
+			this.mixamoPath = params.get("mixamo")!;
+
 		this.mode = match (params.get("mode"), {
 			"studio": () => StudioMode.Editor,
 			"viewer": () => StudioMode.Viewer,
@@ -195,6 +200,12 @@ export class AppElement extends LitElement {
 					skyboxId="test_scene"
 					@scene-state=${this.onSceneStateChange}
 				></sts-bvh-viewer>
+			` : this.mixamoPath ? html`
+				<sts-mixamo-viewer
+					mixamoPath=${this.mixamoPath}
+					skyboxId="test_scene"
+					@scene-state=${this.onSceneStateChange}
+				></sts-mixamo-viewer>
 			` : html`
 				<sts-viewer
 					objectId="sample-room.gltf"