OpenGL shield z-fighting

One more fix that I'd like to see for .22.

Easiest to notice by grabbing the electric shield in GFZ2, then running in the water hallway. Perhaps with video res at 1280x800.

This happens to me on opengl-bsp-cleanup, float_equals, and SRB2_release_2.1.21 on both 32-bit and 64-bit. It does not happen in 2.1.20 (technically it does, but very minimally.) Did anything change since then?