Continue new platform/glium implementations

- Assets, asset manager system
- Rendering system trait
- Kick off glium platform implementation
- And more

raidillon_core/src/scene.rs

@@ -0,0 +1,30 @@
+pub struct Scene {
+    title: String,
+    world: hecs::World,
+    skybox_texture_path: String,
+}
+
+pub struct SceneManager {
+    scenes: Vec<Option<Scene>>,
+    active_index: usize,
+}
+
+impl SceneManager {
+    pub fn current(&self) -> &Scene {
+        self.scenes[self.active_index].as_ref().unwrap()
+    }
+
+    pub fn current_mut(&mut self) -> &mut Scene {
+        self.scenes[self.active_index].as_mut().unwrap()
+    }
+
+    pub fn add_scene(&mut self, scene: Scene) -> usize {
+        let idx = self.scenes.len();
+        self.scenes.push(Some(scene));
+        idx
+    }
+
+    pub fn remove_scene(&mut self, idx: usize) {
+        self.scenes[idx] = None;
+    }
+}