Improve abstraction of the engine

- Improved camera controls
- Introduce new convenience function ecsr.load_mesh_from_gltf
- Abstract out the glium stuff to allow for more backends in the future.
  We're still tied to winit though as it can be used with any of the
  major graphics libraries in the Rust ecosystem.

raidillon_render/src/lib.rs

@@ -3,7 +3,9 @@ pub mod model;
 pub mod gltf_loader;
 pub mod render;
 pub mod ecs_renderer;
+pub mod window;
 
 pub use camera::Camera;
 pub use render::GliumRenderer;
 pub use ecs_renderer::ECSRenderer;
+pub use window::{DisplayHandle, init_window as init_render_window};