Implement pinch zoom emulation to the glutin port

The GLFW port had pinch zoom emulation that could be triggered by
holding control and using the mouse wheel. This was very useful for
testing pinch zoom behavior on desktop machines. This commit
implements this for the glutin port.

ports/glutin/window.rs

@@ -200,11 +200,20 @@ impl Window {
                     WindowEvent::MouseWindowMoveEventClass(Point2D::typed(x as f32, y as f32)));
             }
             Event::MouseWheel(delta) => {
-                match delta {
-                    MouseScrollDelta::LineDelta(dx, dy) => {
-                        self.scroll_window(dx, dy * LINE_HEIGHT);
-                    }
-                    MouseScrollDelta::PixelDelta(dx, dy) => self.scroll_window(dx, dy)
+                let (dx, dy) = match delta {
+                    MouseScrollDelta::LineDelta(dx, dy) => (dx, dy * LINE_HEIGHT),
+                    MouseScrollDelta::PixelDelta(dx, dy) => (dx, dy),
+                };
+
+                if !self.key_modifiers.get().intersects(LEFT_CONTROL | RIGHT_CONTROL) {
+                    self.scroll_window(dx, dy);
+                } else {
+                    let factor = if dy > 0. {
+                        1.1
+                    } else {
+                        1.0 / 1.1
+                    };
+                    self.pinch_zoom(factor);
                 }
             },
             Event::Refresh => {
@@ -225,6 +234,10 @@ impl Window {
         self.key_modifiers.set(modifiers);
     }
 
+    fn pinch_zoom(&self, factor: f32) {
+        self.event_queue.borrow_mut().push(WindowEvent::PinchZoom(factor));
+    }
+
     /// Helper function to send a scroll event.
     fn scroll_window(&self, dx: f32, dy: f32) {
         let mouse_pos = self.mouse_pos.get();