comp_numeric_transition fails spectacularly when counting down

[electrovoyage]

As mentioned in #265, comp_numeric_transition completely fails when counting down. In the screenshot below, the Start Value keyvalue is set to 1 and the End Value keyvalue is 0.

For whatever reason, it not only stops when it's roughly 40% done, as described in #265, but it also starts counting down towards -1 for some reason.

This behaviour was also present in at least one previous version.