Added more metal sign colors

Red w/white border (e.g. "Stop" or similar),
white w/red border (e.g. "Yield", "Do not enter", and similar),
white w/black border (speed limit, other info signs)

renamed and simplified their edge/top/bottom textures since they always have
metal edges anyways (made them all use the same files).  Simplified the code
that creates them.  Minor changes to the code that places them to facilitate
this expansion (also allows for future expansion if necessary).