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)

srb2win_depth-buffer-24.exe