Implement trackpad pinch-to-zoom for plots in egui_web (#333)

This adds a new `zoom_delta` to input.
This is hooked up to ctrl-scroll on egui_web and egui_glium.

Browsers convert trackpad pinch gestures to ctrl-scroll,
so this means you can not pinch-to-zoom plots (on trackpad).

In the future we can support multitouch pinch-to-zoom via the same
`InputState::zoom_factor()` function

egui_demo_lib/src/apps/demo/plot_demo.rs

@@ -90,6 +90,8 @@ impl PlotDemo {
                 ui.checkbox(proportional, "proportional data axes");
             });
         });
+
+        ui.label("Drag to pan, ctrl + scroll to zoom. Double-click to reset view.");
     }
 
     fn circle(&self) -> Curve {