TINYGL: Dirty rects system.

This PR contains an implementation of dirty rects system.
You can find more information behind the design and implementation in the following blog posts: 
http://subr3v.blogspot.co.uk/

Move this down to the while-loop, and consider making the while-loop a for-loop of the: for (it = X.begin(); it != X.end; ++it)-kind for clarity.