Implement Engine, add compile_shader to rendering systems

- Implement the initial version of the run method of the glium platform
- Add asset manager to the context of the renderer. I will probably move that to either the engine or somewhere else later.
- Other unimportant stuff that I'm too lazy to include here. Early stage commit messages don't really matter anyways.

raidillon_core/src/engine.rs

@@ -0,0 +1,6 @@
+use crate::SceneManager;
+
+pub struct Engine {
+    scene_manager: SceneManager,
+    // TODO: Systems
+}

raidillon_core/src/lib.rs

@@ -1,2 +1,5 @@
 mod scene;
-pub use scene::{Scene, SceneManager};
+mod engine;
+
+pub use scene::{Scene, SceneManager};
+pub use engine::Engine;