Make box moar boxxy.
Refactor ui and renderer.

Move teh SDL window and teh OpenGL context, shaders and programs to
engine.c and make them static.

SDL initialization is now handled at engine.c, with teh main function.

Also, remove `struct r_box`, since it didn't add anything to `struct
g_box`;
Add r_game.c
Add player as a member of struct game.
Improve keyboard handling.
Add preliminary walking with keyboard arrows
Fix collisions by rewriting everything right.