FOF slopes level load fix

This fixes a very old glitch with FOF slopes in software mode: the first time you some FOF slopes (most obvious at level load, when the level fades in from black) they appear to be displayed in the wrong place, but then they apparently automatically fix themselves. This was because an internal hasslope variable that marks sectors as having slopes (whether normal or on FOFs inside the sector) isn't set properly until after that first time.

I do vaguely remember a very similar bug in OpenGL in one of Thokker's maps a long time ago, but I haven't tested if anything's changed there because of these fixes.