fixings and improvements

dev:
- add documentation
- main() is more readable

fix:
- unmanaged errors:
  - map parsing: error if color value is >255
  - map parsing: load_textures(): check return value of xml_xpm_file_to_image()
  - main.c: draw_screen(): check return value of mlx_new_image()

other details:
- dev: color_from_rgb(): change signature
- fix: nullize map in map_destroy
- change: change error message for read_all_text() errors
- change: change rotation speed
- change: add and use contant NOOBONUS_SPEED

dev/TODO

@@ -11,4 +11,3 @@ TODO: map parsing: fix segfault if bad texture
 TODO: map parsing: fix when color value > 255
 TODO: what if the pointer goes outside of the window?
 TODO: fix inverted west and east textures
-TODO: fix nobonus speed