- 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/gl/compatibility/gl_20.cpp

@@ -588,6 +588,7 @@ bool FDrawInfo::PutFlatCompat(GLFlat *flat, bool fog)
 	int list = list_indices[masked][foggy];
 	auto newflat = gl_drawinfo->dldrawlists[list].NewFlat();
 	*newflat = *flat;
+	newflat->vboindex = -1;	// don't use the vertex buffer with legacy lights to ensure all passes use the same render logic.
 	return true;
 }