Allow to specify renderOrder on Sprite instances.

This pull-request adds `renderOrder` functionality which  `Mesh` instances already have.

src/renderers/webgl/plugins/SpritePlugin.js

@@ -351,8 +351,12 @@ THREE.SpritePlugin = function ( renderer, sprites ) {
 	}
 
 	function painterSortStable ( a, b ) {
+		
+		if ( a.renderOrder !== b.renderOrder ) {
 
-		if ( a.z !== b.z ) {
+			return a.renderOrder - b.renderOrder;
+
+		} else if ( a.z !== b.z ) {
 
 			return b.z - a.z;