- use an indexed vertex buffer to render the flats.

Right now this has no advantage but it allows optimizing the data, e.g. rendering an entire sector in one go instead of per subsector.

src/p_setup.cpp

@@ -1510,6 +1510,7 @@ void P_LoadSectors (MapData *map, FMissingTextureTracker &missingtex)
 		ss->friction = ORIG_FRICTION;
 		ss->movefactor = ORIG_FRICTION_FACTOR;
 		ss->sectornum = i;
+		ss->ibocount = -1;
 	}
 	delete[] msp;
 }