Duke3D: Main game loop tune-up - redux.

Earlier, I raised !2 (closed) with some changes to the main game loop but had some issues so I closed it. What actually happened was that these changes exacerbated the stuttering issues reported on Windows and led me to find the issue.

Moving this code from here to here has an overall detrimental impact on the game. Moving this code back and playing with r_glfinish 1 is as smooth as butter.