fixed: R_PointOnSideSlow had precision issues with very short lines.

When this function was originally written there was no possibility of fractional vertex coordinates so it threw away the fractional parts of the node's directional vector (which in the original nodes was always 0.)
Now, with UDMF and high precision vertices this no longer works and the loss of significant parts of their value caused this code to produce erroneous results if the linedefs were only a few map units long and using fractional positions.