scrolling is not captured on web mobile

[rukai]

On the web on desktop devices, scrolling the mouse while the cursor is over the winit window will not scroll the webpage.
This is good and desirable.

However one finger dragging over the winit window on a mobile device will scroll the webpage.
This is bad, particularly on mobile as the cursor position events reports strange movement due to the page scroll negating the movement of the 'cursor'.

Libraries like three-js handle both of these cases by preventing scrolling.

Possibly it will be fixed by just handling the relevant events in #1945
Or maybe we need to do more.

[rukai]

After experimenting with #1945 it doesnt yet solve this problem but it can be adjusted to fix it with the addition of calls to event.prevent_default().

This change is enough to get it working.

We would obviously need to rewrite to not use those mem::forgets though.

However thinking about this there are valid use cases for allowing scrolling while the canvas is focused.
You could use a winit for rendering a completely noninteractive animation or you could have a UI that never needs to click and drag. In these cases being able to scroll freely on the web page is more natural for the user.

As such maybe the solution here is to move all the event.prevent_default() callbacks behind a web_disable_scroll() method.

However allowing scrolling by default is quite problematic as any sort of click and drag functionality ends up completely broken on mobile.
So we would probably be better off defaulting to disabling web scroll and providing a web_enable_scroll() method to enable it for the users who know they really want it.