Fix color attribute initialization of InstancedMesh (mrdoob#22171)

* Fix color attribute initialization in InstancedMesh

* Use matrix buffer size instead of storing maxcount

Author: FoolHen

src/objects/InstancedMesh.js

@@ -95,7 +95,7 @@ class InstancedMesh extends Mesh {
 
 		if ( this.instanceColor === null ) {
 
-			this.instanceColor = new BufferAttribute( new Float32Array( this.count * 3 ), 3 );
+			this.instanceColor = new BufferAttribute( new Float32Array( this.instanceMatrix.count * 3 ), 3 );
 
 		}