Render midtextures on two-sided lines with a z-buffer offset

This will fix z-fighting issues when midtextures overlap with FOFs. This issue is shown in the second screenshot in #190 (closed). That issue can be closed when this and !1361 (merged) are merged.

I also have a separate branch where the effects of this fix can be compared with the gr_ztest command here: https://git.do.srb2.org/Hannu_Hanhi/SRB2/tree/ogl-midtexture-zfix-test