Enable multitouch in Glutin.

This tells Glutin's Android and iOS back-ends to send events for more than one
pointer at a time.

ports/glutin/window.rs

@@ -93,6 +93,7 @@ impl Window {
                             .with_gl(Window::gl_version())
                             .with_visibility(is_foreground)
                             .with_parent(parent)
+                            .with_multitouch()
                             .build()
                             .unwrap();