This gives the resultant point the correct sign. Though the projective
divide would take care of the sign, this makes reading the point a
little less confusing (still need to sort out automatic blade reversals
for the likes of e31).
As the dot product is a metric product, using the metric is vital to
getting the correct results. This fixes the calculation of the closest
point on a line to a point other than the origin (and a whole pile of
other issues, I imagine).
