vsync support

-works on windows
-may not work on X11, if so please fix
-OSX does not seem to support disabling vsync

drivers/gl_context/context_gl.h

@@ -52,7 +52,11 @@ public:
 	virtual void swap_buffers()=0;
 	
 	virtual Error initialize()=0;
-	
+
+	virtual void set_use_vsync(bool p_use)=0;
+	virtual bool is_using_vsync() const=0;
+
+
 	
 	
 	ContextGL();