Wow, found out a bit more detail about how rendering is done in MC2.
In tgl.cpp there is usually Update and Render methods. So Update updates and transforms shapes AND at the same time allocates triangles in masterTextureNodes accordingly.
And if there should not be any texture then nodeId == 0xffffffff.
Later in Render method, necessary vertices are added. Somehow, those methods should be in sync, so that we add exactly same amount of triangles that were allocated in Update method.
In a same way, nodeId == 0xffffffff if no texture should be used, also in this case masterTextureNodes[0] is used to store triangles info.
Found all that when trying to figure out why windows sometimes become transparent and sometimes not. So have to make sure that texture handle 0 means no texture and just add fake texture when renderer is initialized, so that no valid texture wil have handle 0. Also have to figure out, what does it mean when terrain is drawn with texture handle 0, should those triangles be drawn or can we just skip them (apparently skipping them seems to be ok, but have to make a thorough check).
And finally - even if I’ve found a problem with windows, it is still a question why they are drawn only in some cases and in other cases, they are not drawn.