Request 24-bit depth buffer from SDL
Should fix problems caused by 16-bit depth buffer usage.
Needs testing. I have no computers that give a 16-bit depth buffer to SRB2, so I have no way to know if this actually fixes the issue.
The issue in the first gif in #424 (closed) (and many other z-fighting issues) should be fixed by this.
Testing build: (2.2.9 netgame compatible)