Papersprite dropshadows no longer askance

Fix dropshadows of papersprites drifting depending on angle relative to camera. Discovered in Kart internal as a regression on the paper item drops and ported back.

No executable or example file, sorry. Not in vanilla work mode right now, this commit was done using the web IDE (but it's a line-for-line match for the original commit). (closes #409 (closed))