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.
Actually, I'm partially wrong there; fuzzie points out that we don't have an operator[] in List at all, so this probably means the current version of the CONSISTENCY_CHECK code is broken?