egui support

- New Resource, EguiQueue: Utility to queue egui ui builders
- New Rendering System, EguiRenderer: Utilizes a modified egui_glium
  library to render egui UI
- Adjusted RenderingSystem trait and RenderingContext structure to
  provide event_loop and egui_queue.
- Various minor adjusments

core/src/egui_queue.rs

@@ -0,0 +1,22 @@
+pub struct EguiQueue {
+    pub queue_vec: Vec<Box<dyn FnOnce(&egui::Context) + Send>>,
+}
+
+impl EguiQueue {
+    pub fn new() -> Self {
+        Self { queue_vec: Vec::new() }
+    }
+
+    pub fn queue(&mut self, func: impl FnOnce(&egui::Context) + Send + 'static) {
+        self.queue_vec.push(Box::new(func));
+    }
+
+    pub fn clear(&mut self) {
+        self.queue_vec.clear()
+    }
+
+    pub fn run(&mut self, ctx: &egui::Context) {
+        self.queue_vec.drain(..).for_each(|func| func(ctx));
+        self.clear();
+    }
+}

core/src/engine.rs

@@ -1,5 +1,3 @@
-use std::cell::RefCell;
-use std::rc::Rc;
 use crate::DebugUIBuffer;
 use crate::scene::Scene;
 
@@ -14,5 +12,4 @@ pub trait EngineTrait {
     fn current_scene(&self) -> &Scene;
     fn get_debug_ui_buffer(&self) -> &DebugUIBuffer;
     fn reset_debug_ui_buffer(&mut self);
-    // fn scene_and_debug_ui_buffer_mut(&mut self) -> (&mut Self::Scene, &DebugUIBuffer);
 }

core/src/lib.rs

@@ -3,5 +3,7 @@ pub mod debug_ui;
 pub mod time;
 pub mod utils;
 pub mod scene;
+mod egui_queue;
 
 pub use debug_ui::*;
+pub use egui_queue::EguiQueue;