reverting to automatic cmake package finding

SDL_gfx has replaced SDL_gfxPrimitives, so some slight modifications
were needed. I've adapted the official FindSDL_image.cmake module and
included it here. (It's pretty trivial, and will likely be accepted
upstream if submitted.)

I've tested it on Mac OS X with Homebrew's SDL, SDL_image, and SDL_gfx
packages installed. It appears to compile and link everything correctly,
but crashes immediately on running.