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.
Why is this constant defined from another constant only when a third constant is also defined? (As in, why not use MAX_INPUT_RECTS to begin with?)