Object flickering

[stephen-hqxu]

Problem

When new chunks are being loaded, sometimes, some objects, mostly the terrain, turns black for a couple of frames before returning to normal.

Testing shows it flickers more frequently when the rendering task is heavy. To reproduce, set the FPS limit to as high as possible and turn up the atmopheric scattering quality to very high, and the objects flickers constantly.

[stephen-hqxu]

Fixed

It is as simple as adding a glFinish() function before swapping buffer, because I was assuming swap does implicit sync but actually it does not.