Opengl improvements backport

Partial backport of some cleanup and fixes I made in the OpenGL code for 2.2, there may or may not be more to come. Among other things, I've cleaned up hw_md2.c and hw_trick.c

As for fixes, gr_correcttricks on should no longer break flats on slopes. We had that happen in 2.2's GFZ2 if you've been reading the commit descriptions, lol.

EDIT: Also backported the pv1/pv2 pointers I created for seg rendering in OpenGL. Here if my original description of pv1/pv2 addition from the internal MR I made:

hw_bsp.c's AdjustSegs no longer assigns polyvertex_t (OpenGL's own kind of vertex for rendering purposes) data to v1/v2 of segs directly, but new pointers pv1/pv2 for polyvertex_t data specifically. These pv1/pv2 pointers are then used everywhere for OpenGL's seg rendering instead of v1/v2. This means that the actual seg vertexes should now be consistent in both renderers, and in theory the original R_IsPointInSubsector could work in OpenGL now if we reverted to that.