Invert Y axis so it points down (front)

This is what Desert Strike uses and with this change there will be no
need for extra coordinate conversion
Implement and use simple camera
Implement and use simple ground renderer, move ground painting there
Add casts of 3D things to 2D counterparts
Add game field dimensions
Extend camera with viewport information

Camera is now able to transform game coordinates into screen space
Draw sector grid in GroundRenderer
Axe unused Renderer::Update
Add convenience methods to reset z coordinate of Vector3
Rework Renderer to use camera
Implement unary - for vectors
Implement some ugly heli movement
Update libSDL2pp
Add minimal level decoder
Bring back SDL2pp::Renderer to Renderer

This allows drawing non-sprite objects
Allow geting Renderer reference from SpriteManager
Introduce Building object stub
Introduce LevelLoader

This is essentially a Game factory which populates newly created Game
with objects present on the level