Dropshadow crashes in software renderer

Writing this here so I don't forget, but I have recently managed to confirm drop shadows sometimes crash the game! Specifically, this is due to frac being negative in the column drawers, which seems to be caused by a large negative value set for dc_texturemid (a sort of reference point for the vertical position to start drawing from in the actual sprite's pixel data) which in turn seems to come from a tiny y scale set when working out the shadow's projection.

Not sure yet how to fix this yet though, but hopefully we'll get there.