Event: Normalize mouse event properties in drag events

DragEvent is a superset of MouseEvent, so we want to fix up mouse
properties like pageX and pageY.

Fixes gh-1925
(cherry picked from commit 389b2ab3b93bfd68ca6c6153a43e11d93ab9ec71)

src/event.js

@@ -12,7 +12,7 @@ define([
 
 var rformElems = /^(?:input|select|textarea)$/i,
 	rkeyEvent = /^key/,
-	rmouseEvent = /^(?:mouse|pointer|contextmenu)|click/,
+	rmouseEvent = /^(?:mouse|pointer|contextmenu|drag)|click/,
 	rfocusMorph = /^(?:focusinfocus|focusoutblur)$/,
 	rtypenamespace = /^([^.]*)(?:\.(.+)|)/;