Duke3D: No screenfade effect when exiting a level in Polymost/Polymer

In the Classic Mode renderer, after hitting a nukebutton, the game slowly fades out to black, and fades back in on the bonus screen. This also occurs when beating a boss monster. Note that this means that the cutscenes more easily be accidentally skipped, as the fadeout normally prevents inputs from the player being carried over to cutscenes or the bonus screen.

In Polymost or Polymer, this transition is missing, and the game instead immediately jumps to the bonus screen. Note that this problem is very old, even occurring in eduke32 builds from 2007, before the versioning system was introduced.

(Issue originally raised by: https://forums.duke4.net/topic/11841-no-fade-effect-in-polymost/)