* The exact location of particle birth wasn't cached if cache step was greater than 1, so the interpolation from an unborn particle to a living one wasn't accurate at all.
* Although not ideal, the current solution is to copy the birth location to the previous cached frame so it's always exact. This should work properly for memory & disk cache.
* This fix shouldn't cause any problems, but exhaustive tests of caching are difficult so be sure to let me know if there's strange behavior.
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