Add ViewportCommand to winit backend

[thequver]

Is your feature request related to a problem? Please describe. While egui-winit official integration exist, would be good to have ctx.send_viewport_cmd() working "out of the box" Currently it does nothing

[emilk]

The stand-alone egui-winit could use some love.

Currently it is up to the user to call egui_winit::process_viewport_commands.