OpenGL: Fix linedef type 10 not culling FOFs (Closes #438)

The title is exactly what this merge request does. I fixed linedef type 10 not culling FOFs so that linedef type 10 can be used as a workaround to Issue #438 (closed) since it increases performance and does not require the floor of the FOF for each layer to be F_SKY1.

Test file: cullfixtest.wad