Close *all* resources before exit

[PeterBowman]

As a follow-up to #79, I'd suggest closing TTF- and IMG-related resources, too. Similarly to SDL_Init() and SDL_Quit(), the following functions apply here:

• TTF_Quit(): https://www.libsdl.org/projects/SDL_ttf/docs/SDL_ttf_10.html#SEC10
• IMG_Quit(): https://www.libsdl.org/projects/SDL_image/docs/SDL_image_9.html#SEC9

If I understand it correctly, both should be called immediately before SDL_Quit(). We should have now a proper place to call these functions from: rd::SDLScreenManager::cleanupSDL.

[PeterBowman]

Started with 2da58f1. Pasting atexit(3) function description for future reference (bolding mine):

The atexit() function registers the given function to be called at normal process termination, either via exit(3) or via return from the program's main(). Functions so registered are called in the reverse order of their registration; no arguments are passed.

[PeterBowman]

Merged. Note that it's safe to call *_Quit functions even if no modules have been initialized yet.

[jgvictores]

@PeterBowman Thanks! Great work!!

[David-Estevez]

Cool! Thanks, @PeterBowman !!