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.

engine/src/system.rs

@@ -15,10 +15,10 @@ pub struct SystemContext<'a> {
 
 pub trait System {
     /// Initialize the system.
-    fn initialize(&mut self);
+    fn initialize(&mut self) {}
     /// Spawn the first entities of the world.
-    fn load_world(&mut self, ctx: &mut SystemContext);
-    fn update(&mut self, ctx: &mut SystemContext);
+    fn load_world(&mut self, ctx: &mut SystemContext) {}
+    fn update(&mut self, ctx: &mut SystemContext) {}
 }
 
 pub type SystemID = &'static str;