for consistency in size (per MUTCD for multi-lane conventional roads). This
had the side benefit of reducing the number of unique models, also.
Some signs are bigger than before, some are smaller. All of the reworked
models are now 50 mm thick (a bit thinner than before).
Also, consistent image resolution -- all reworked signs' graphics are
256px, which means most of them are lower resolution than before, but not
enough to make a big impact in quality.
Also, split the pole mount and the signs' eging from the main texture,
otherwise positions of these things in a sign image are unpredictable.
Got rid of all the entity UV guides images, as they're all invalid now.
