Bat + balls fixed, snow refined, skybox added

...able/special_drawable/skybox_drawable/skybox_drawable.cpp

...able/special_drawable/skybox_drawable/skybox_drawable.hpp

projet-code/scenes_inf443/base/assets/skybox.jpg

projet-code/scenes_inf443/base/assets/skybox_debug.png

...e/shaders/mesh_multi_texture/mesh_multi_texture.frag.glsl

projet-code/scenes_inf443/base/src/ball.cpp

projet-code/scenes_inf443/base/src/scene.cpp

projet-code/scenes_inf443/base/src/scene.hpp

projet-code/scenes_inf443/base/src/terrain.cpp