OpenGL new clipping backport

Backport of 2.2's new clipping code for OpenGL.

Original description from the internal MR I made for this (along with the changes in the now-merged OpenGL improvements branch):

EXPERIMENTAL NEW CLIPPING CODE - this is the thing I need people to check out in particular. I have made efforts to port SRB2CB's clipping code, which is in turn from PrBoom+, in order to try and improve the speed of OpenGL rendering. While I am not sure if I succeeded in doing this, this also fixes rendering glitches with translucent walls (such as the waterfall at the start of GFZ1).