Allow to specify the Texture channel for the lightmap

Newer three.js versions support multiple uv attributes for gltf models. This introduces the need to specify which uv attributes is the one relevant for our lightmap texture.

See mrdoob/three.js#25721

model-utils.js

@@ -13,13 +13,18 @@ AFRAME.registerComponent('lightmap', {
     },
     basis: {
       default: false
+    },
+    channel: {
+      type: 'int',
+      default: 1
     }
   },
   init() {
 
     const src = typeof this.data.src === 'string' ? this.data.src : this.data.src.src;
     const texture = new THREE.TextureLoader().load(src);
     texture.flipY = false;
+    texture.channel = this.data.channel;
     this.texture = texture;
 
     this.el.addEventListener('object3dset', this.update.bind(this));
@@ -103,4 +108,4 @@ AFRAME.registerComponent('no-tonemapping', {
       }
     }.bind(this));
   }
-});
+});