Fix for performance bug when displaying underground grund_t

[ceeac]

Previously underground grund_t would always be drawn and not only when dirty.

[prissi]

I think this was needed to avoid glitches. But since the code went to several improvement, maybe it has become superflous. I will futher investigate this.

[prissi]

Thanks, fixed in 8833. That routine had a parameter dirty in the old days, so it did make sense then. Not now anymore, of course.