Fixed text_extents width bug.

Sorry; I left a bug in here last time around. Width was having start_x
subtracted from it inappropriately. Fixed.

As it turns out, it appears that start_x and start_y are irrelevant parameters
to this function. I didn't remove them though as they're in vgft.h and it's not
clear whether external clients are allowed access to this interface. I marked
them as unused.