added entities and fixes

README.md

cmake/Cache.cmake

resources/images/enemy3.png

src/game/game.cpp

src/game/renderer/sdl_resources.cpp

src/game/renderer/sdl_resources.h

src/game/state/CMakeLists.txt

src/game/state/entities.cpp

src/game/state/entities.h

src/game/state/shooter.cpp

src/game/state/shooter.h

src/game/state/state.cpp

src/game/state/state.h