Add "draw-on-top" overlay for OS X

[mkrautz]

All full screen games on OS X are supposed to not "capture" the screen when doing full screen -- but instead use a full screen borderless window. The compositor then optimizes for this scenario.

It would be nice if we, instead of doing injection and hooking, could simply draw a translucent rect with the overlay on top full screen (and/or windowed) apps. The placement of the rect would update when the window changed position (i.e. Mumble would query for the position of the window, determine if it's changed, change the offset it is drawing the overlay at.).

If no overlay-enabled application is running, Mumble would simply hide the "draw-on-top" overlay.

This approach brings some challenges, such as: how do we draw on top of everything?
AFAIR, OS X defines a special window level for full screen apps. It should be possible to simply one-up this level, to draw on top of full screen games.

Another challenge is OS X's built in fullscreen feature. Essentially, full screen programs run in their own "space" (virtual desktop). Mumble will have to take this into consideration.

An application we can take inspiration from is xScope (http://xscopeapp.com/). It is a measuring tool for designers. It can draw on top of a lot things, with an on-screen ruler, among other things.