Fix logic bug causing oil drops to rotate. Really fixed now!

This corrects 211eb80c0aad.
Minor extensions to script interface:

- quad_setRepeatTexture()
- quad_setRepeatScale()
- quad_isRepeatTexture()

(bone, entity, ... as well)

- getOldDT() - return unmodified dt
- getDT() - as passed to update(), for complicated edge cases
            where dt is unavailable but required.