fixed bug with extremely slow envelopes when using "request.step_delay: 0"

Using a step delay of zero for envelope style requests (and also in-game
envelopes) resulted in extremely slow envelope animations (instead of very
fast animations, as could be expected), because "SkipUntilDelayReached()"
won't skip any frames in this case (which should probably be corrected,
too). This is fixed by never using a step delay value less than "1".