This implements the SDL_rect.h interface, except for SDL_EnclosePoints()

and SDL_IntersectRectAndLine(), which need some features cl-autowrap
doesn't have yet. The API was made somewhat lispy, but not overly so.
I tried to avoid unnecessary memory allocations in the codes.

I'm still a little wary of memory leaks with the cl-autowrap interface, so I would
request close attention to that.

If there is anything you'd like me to fix, please let me know!