- render sector planes in one draw call.

On a fast and modern graphics card this is a lot faster than doing it per subsector but it may not be without drawbacks on older hardware so it will require some testing on older hardware.
For me Frozen Time's view over the bridge went from 46 fps to 51 fps with this change, the time saved was roughly 2 ms.

src/hwrenderer/data/flatvertices.cpp

@@ -165,6 +165,7 @@ int FFlatVertexGenerator::CreateIndexedSectorVertices(sector_t *sec, const secpl
 		subsector_t *sub = sec->subsectors[j];
 		CreateIndexedSubsectorVertices(sub, plane, floor, vi, gen);
 	}
+	sec->ibocount = ibo_data.Size() - rt;
 	return rt;
 }