DesktopStreamer not retina-aware

[eile]

The desktop streamer on OS X only lets me select the non-retina resolution, but then streams out the retina resolution. The result is that I can at most stream 1/4 of the screen.

I'll start looking into this.

[eile]

Needs not-yet-release Qt4.8.x: https://codereview.qt-project.org/#change,54636

Qt info on HighDPI: http://blog.qt.digia.com/blog/2013/04/25/retina-display-support-for-mac-os-ios-and-x11/

Will implement hard-coded hack for now, to be replaced with automatic detection once the above is released.