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/Cargo.toml

@@ -12,3 +12,4 @@ glutin = { version = "0.32.3", default-features = false }
 hecs   = "0.10.5"
 image  = "0.25.6"
 raidillon_ecs = { path = "../raidillon_ecs" }
+winit = "0.30"