Add necessary bits to allow signs to be placed onto horizontal poles

Add necessary bits to allow signs to be placed onto horizontal poles
(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.