Geometry: groupsNeedUpdate support. Should fix #6694.

src/core/Geometry.js

@@ -45,8 +45,6 @@ THREE.Geometry = function () {
 	this.colorsNeedUpdate = false;
 	this.lineDistancesNeedUpdate = false;
 
-	this.groupsNeedUpdate = false;
-
 };
 
 THREE.Geometry.prototype = {
@@ -1071,6 +1069,14 @@ THREE.Geometry.prototype = {
 
 		this.dispatchEvent( { type: 'dispose' } );
 
+	},
+
+	// Backwards compatibility
+
+	set groupsNeedUpdate ( value ) {
+
+		if ( value === true ) this.dispose();
+
 	}
 
 };