texture: add internalFormat to copy() method

Author: DavidPeicho

src/textures/Texture.js

@@ -102,6 +102,7 @@ Texture.prototype = Object.assign( Object.create( EventDispatcher.prototype ), {
 		this.anisotropy = source.anisotropy;
 
 		this.format = source.format;
+		this.internalFormat = source.internalFormat;
 		this.type = source.type;
 
 		this.offset.copy( source.offset );