Set fixed framerate [fix bug #3095]

If a window loses focus in OSX, it no longer receives VSYNC events. If ofSetFrameRate() has not been called, it will no longer be frame rate limited. This can cause unwanted behaviour, such as the application maxing out CPU usage.

See: https://github.com/openframeworks/openFrameworks/issues/3095
Merge pull request #53 from JobLeonard/patch-1

Set fixed framerate [fix bug #3095]