Fix imgui renderer once and for all

Finally solved the problems with the imgui renderer after a long chat with clankers. Fixed some other stuff as well.

Reminder to keep the rendered_this_frame check as that's what solved it. Probably a deeper issue down there that caused us to render twice, but whatever.

glium_platform/src/system.rs

@@ -19,6 +19,8 @@ pub struct RenderingContext<'a> {
 /// The internal "rendering system" trait of glium_platform.
 /// This is unrelated to the main System trait in core.
 pub trait RenderingSystem {
+    fn handle_event(&mut self, window: &mut glium::winit::window::Window, event: winit::event::Event<()>) {}
+    fn prepare_frame(&mut self, window: &mut glium::winit::window::Window) {}
     fn render(&mut self, ctx: &mut RenderingContext);
     fn initialize(display: &Display<WindowSurface>, window: &glium::winit::window::Window) -> Self where Self: Sized;
 }