Use the raster rendering engine by default on OSX

Use the raster rendering engine by default on OSX

Seems like using the raster engine fixes performance issues on OSX,
and there seem to be no noticeable side-effects compared to native
rendering. So let's use this by default.

If you notice issues, please let us know. Also, you can always enforce
native rendering via command line:

open Quassel\ Client.app -W --args -graphicssystem native

Thanks to rikai for figuring this out!