Fix visplane getting allocated twice

Looks, compiles, and plays fine... if anyone else who's more experienced with this part of the code knows a reason for allocating space for 2 then let me know.

This might've been an early Legacy-ism (or other related source port) for somehow reusing the same plane memory for both a floor and a ceiling, but it doesn't sound compatible with 3D floor code, and I can't seem to find any other source port that does the same. Modern Legacy allocates one, Boom allocates one, Eternity allocates one, PrBoom allocates one, GZDoom allocates one, and Doom uses a static array for visplanes, so it doesn't matter here. So this is safe to do.

merged

mentioned in commit 5f70d3e1