Code review comment for lp:~alan-griffiths/mir/fix-1532202

Aside from the above comment, my reason for not liking regional damage is based on previous experience in Compiz and even in mir_proving_server. Accurately tracking damage when you might be painting things using free-form OpenGL (not bound by any surface extents) like shadows is just too much of a pain to maintain. And on low-end systems can become a CPU bottleneck.

Put another way, given two monitors A and B, your window may only exist on A, but you really have to redraw both A and B in case shadows or some other side-effect creeps across.