Corrected algorithm of the definition atlas size.
======= New Features =======
- now Atlas class don't storage his width and height explicitly, it storage
  indexes of the atlas width and height for size array;

======= Bugfixes =======
- methods for settings the atlas width and height was incorrect work;
- if choose "Add textures", "Packing textures", "Add textures" and "Packing
  textures" again, application terminate with appearance a runtime error.
  Now, after clicked "Packing textures", action "Add textures" sets disabled.

======= ToDo in Future =======
- need to decide how to define the atlas size when textures cannot packing
  to atlas with current size.

Signed-off-by: Oleg Matiyuk <olgerd27@gmail.com>