Code review comment for lp:~alan-griffiths/mir/fix-1447886

Feels like an improvement. I don't want to be a blocker on this for too long, but things worth noting:

New bug: Dragging a corner past the window's minimum size and back again results in the cursor losing its original position within the window. The cursor keeping the same relative position within the window for the duration of the resize feels most professional and least frustrating. Just try it in Compiz/Unity7 to get a feel for what should happen. Admittedly mir_proving_server has the same bug right now.

Old bug: I just noticed you're dividing the window into quarters instead of thirds. So grabbing a window from the middle of an edge incorrectly resizes two axes instead of one. See Compiz/Unity or mir_proving_server for examples of what should happen. This one would represent a regression for mir_proving_server, but also isn't the bug being fixed here. Just a recommended prerequisite before abolishing DefaultWindowManager.