Fix an issue with the celt_trader because the triangle was too far below the ground and that caused flickering issues. Ideally at some point we will remove the limitation for prop only meshes to have triangles. While at it restore the units back to meters.

Thread: https://wildfiregames.com/forum/index.php?/topic/28310-flickering/page/3/&tab=comments#comment-398652
Reported by: Mr.lie
Debugging with: @vladislavbelov