TINYGL: glPolygonStipple seems to work when looking at one side only

[neuromancer]

When using the combination between polygon stipple and polygon offset in the Freescape games (which is used to create the patterns): the triangles tend to be invisible, when you change the perspective a little bit. But there is something else, that is curious. In a symmetrical room, one side looks good and the other looks broken. (check the screenshots). Reversing the perspective changes the sides, so it looks like the rendering is correct is always in the same side so I suspect this is an issue in TinyGL since the same code works fine for OpenGL/shaders.

[neuromancer]

Something else that was proposed for this issue in Discord:

do you think it could be because there is a missing statement that updates the zbuffer in FrameBuffer::fillTriangle ?
Perhaps in the update_right branch? There is an update to the zbuffer in update_left, but nothing in the other branch: ​https://github.com/scummvm/scummvm/blob/494ec37f20e3560d5e82829612086262401720e2/graphics/tinygl/ztriangle.cpp#L380-L444