[UDMF-Full] Equation-based slope overflow error.

Equation-based slopes have an overflow error when at high map coordinates.

Moving the stage inwards fixes the problem.

However, it then breaks every other equation-based slope in the stage, as they are too far in negative coordinates.

I've moved the stage around every way possible. It is too large for there to be any "sweet spot" that works. I assume these are interger limit issues.