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/

For readability, you should name the params in the doxygen-comments too, also the style we've used earlier is:
/**
 * comments
 * comments
 */
Note the leading stars on each line, aligned with the first star on the first line of the comment block.