Implement adaptive VSync

[sven-n]

Currently the target framework is achieved by a combination of thread sleep and spinning a while(true). This is not optimal, since thread sleep times are very very inaccurate and spinning is wasting cpu cycles.
This should be changed to use (adaptive) vsync like the most games can.

More info:

• https://www.khronos.org/opengl/wiki/Swap_Interval
• https://stackoverflow.com/questions/589064/how-to-enable-vertical-sync-in-opengl

[sven-n]

ae4c6cd