BUG: Incorrect lightmap rendering

[dsvensson]

This bug has been around for quite some time (guess years). Mostly noticed by mappers who has been working around it by for example adjusting texture scale by an invisible amount, or adjusting texture offset. The same maps render correctly in FTE.

Black bottom part of the texture:

Large black parts of the wall:

The bottom of the small box has black parts to the left, and light parts to the right:

Feels like a precision bug, be it a data type resolution or rounding issue.

According to lurq it might be related to negative texture offsets. Will attach a sample map if I manage to reproduce it with a small map. Have maps with the issues to share without filling up this repo's space.