(both wooden and steel signs)
only streets mod's "bigpole" supported for now.
Model changes:
To make a sign support horizontal mounting, make a single "_onpole"
model, with two copies of the pole mount, one rotated by 90°. Make sure
the second copy appears has its own material entry, and that it is last
in the materials list in the exported .obj (`grep "g " file.obj` to
check).
When registered, signs_lib will set the "_onpole" and "_onpole_horiz"
tiles{} to reveal one mount or the other: entry 3 will expose the
vertical mount, entry 4 is the horizontal copy, with the opposite entry
being set to the "blank" image.
