Rewrite F_SkyScroll to fix overflows

Rewrites the F_SkyScroll function to fix integer overflows and, by extension, also avoid occasional holes with negative scroll speeds better (fixes #1008 (closed)). The overflows also happened prior to Uncapped, but Uncapped (probably) made it happen 65536 times faster.
Compared to !1983 (closed), this merge request is much more "invasive", but eliminates the root cause of the problem.

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TL;DR: Fixed menu background images sometimes acting strange