Fix VP9-alpha transparency: drop sRGB on video texture, premultipliedAlpha off

The WebM carries real alpha (alpha_mode=1) but rendered opaque because the
VideoTexture was forced to SRGBColorSpace and the material assumed
premultiplied alpha, crushing transparent regions to black. Removing the
colorspace override and setting premultipliedAlpha:false keys the black out.

public/preview.html

@@ -215,9 +215,16 @@
         vid.muted = true; vid.loop = true; vid.playsInline = true; vid.autoplay = true; vid.preload = 'auto';
         vid.src = webm;
         const tex = new THREE.VideoTexture(vid);
-        tex.colorSpace = THREE.SRGBColorSpace;
+        // VP9-alpha WebMs carry straight (non-premultiplied) alpha. Leave the
+        // texture in the default (linear/no-color-conversion) space — forcing
+        // SRGBColorSpace here makes three.js crush the alpha to black. The
+        // material below blends on the video's own alpha with premultipliedAlpha
+        // off so transparent regions stay see-through.
         tex.minFilter = THREE.LinearFilter; tex.magFilter = THREE.LinearFilter; tex.generateMipmaps = false;
-        const mat = new THREE.MeshBasicMaterial({ map: tex, transparent: true, side: THREE.DoubleSide, depthWrite: false });
+        const mat = new THREE.MeshBasicMaterial({
+          map: tex, transparent: true, side: THREE.DoubleSide,
+          depthWrite: false, premultipliedAlpha: false,
+        });
         mesh = new THREE.Mesh(new THREE.PlaneGeometry(1, 1), mat);
         vid.onerror = () => {
           if (group.userData.fell || !image) return;