WME: Multirect 2014

2014, prettified, simplified version of https://github.com/scummvm/scummvm/pull/392

Provides the ability to keep track of multiple dirty rects in Wintermute, saving on blitting.

Unfortunately, with lots of rects if becomes useless and we fall back to the usual single rect approach.
You shouldn't use size() to check if a list is empty, but !empty().

(And in this specific case you should just iterate over the list, delete the entries, and do a clear() at the end.)