Continue new platform/glium implementations

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

raidillon_platform/src/assets.rs

@@ -1,11 +1,11 @@
 use std::path::Path;
 
-pub type ModelHandle = usize;
-
 /// The asset manager trait of Raidillon.
 pub trait AssetManager {
+    type Model;
     /// Loads a gltf model to VRAM.
-    fn load_gltf(&mut self, path: &Path) -> ModelHandle;
+    fn load_gltf(&mut self, path: &Path);
     /// Unloads the loaded model from VRAM.
-    fn unload_model(&mut self, model_handle: ModelHandle);
+    fn unload_model(&mut self, path: &Path);
+    fn get_model(&mut self, path: &Path) -> &Self::Model;
 }