Add z order for the Map
Add Map class in Core to manage a tmx map and its offset
Add useful methods to Core::Map
First attempt at drawing the map

No optimizations for drawing only the visible tiles.
Fix typos
Tile with global ID 0 should not be drawn
Require the new classes and the tmx gem
Begin changing the loader so it could load the map as well
Make the map drawing barely work
Remove the restriction that Core::Map should contain Tmx::Map instance
Remove commented code
Update the instantiation of the Core::Map
Revert the changes to Core::Map
Add background sizes in Core
Add `size` parameter in the Core::LoopingScene initializer
Add #=== to Utils::Direction
Reorder requires because Core depends on Utils
Core should be responsible for resetting the offset
Add the map assets
Fix Velocity#convert_to_basis and Direction#convert_to_basis