WebGLRenderTarget: Added setSize().

Albeit renderTargets can't currently be resized.

src/renderers/WebGLRenderTarget.js

@@ -37,6 +37,13 @@ THREE.WebGLRenderTarget.prototype = {
 
 	constructor: THREE.WebGLRenderTarget,
 
+	setSize: function ( width, height ) {
+
+		this.width = width;
+		this.height = height;
+
+	},
+
 	clone: function () {
 
 		var tmp = new THREE.WebGLRenderTarget( this.width, this.height );