- 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/data/gl_vertexbuffer.h

@@ -95,6 +95,7 @@ public:
 
 class FFlatVertexBuffer : public FVertexBuffer, public FFlatVertexGenerator
 {
+	unsigned int ibo_id;
 	FFlatVertex *map;
 	unsigned int mIndex;
 	std::atomic<unsigned int> mCurIndex;
@@ -177,6 +178,11 @@ public:
 
 #endif
 
+	uint32_t *GetIndexPointer() const
+	{
+		return ibo_id == 0 ? &ibo_data[0] : nullptr;
+	}
+
 	void CheckPlanes(sector_t *sector)
 	{
 		FFlatVertexGenerator::CheckPlanes(sector, map);