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Add keyboard input class
Use input class in game loop
Add player class.
It moves! :o
Clamp velocity to a max speed.
Use slowdown factor for deceleration.
Never completely stop the player; let slowdown handle it.
Fix slowdown

Slowdown should only be applied when the user isn't trying to move.
Clear the screen before each frame.

This gets rid of artifacts which would get left on the screen if a sprite is
moving too fast.
Sprite cache.
Let sprite_cache manage lifetime of SDL_Surfaces.

Sprite will no longer free its own SDL_Surface.
Use "in" properly.
Add SpriteState and comparison operator
Move SpriteState to top of file and out of class
opCmp needs to be const
Use SpriteState to pick which sprite to use.
Remove obsolete if
Add method to initialize sprites.
Add different sprites for each SpriteState.