Use `piston::input::Input` as the `Raw` event type rather than `piston::input::Event`

[mitchmindtree]

Conrod only needs to know about the Input events, the other of the piston::input::Event variants (Render, AfterRender, Update, Idle) are not used.

I think the Render variant is currently used to get the initial window dimensions, however this is a little hacky and I think we'd be better off requiring that conrod is constructed with some initial window size and then monitors the Input::Resize event for changes to window size.

[bvssvni]

Notice that resize events doesn't work in Glutin for some platforms: rust-windowing/glutin#445 (OSX) and rust-windowing/glutin#799 (Win).

One way to work around this is to extract the size directly from the window and check whether it has changed.

[mitchmindtree]

@bvssvni yeah thanks for the heads up! I've run into this a couple times before and spent some time trying to get it working as an Event in glutin a while back with no luck, so it's fresh in my mind 😜