graphics: high-color water
graphics: make a high-color DIRT tile image
graphics: use a transparent layer to implement wires
graphics: experiment with new forest graphics (work in progress)
graphics: more work on hi-color forest
graphics: experiment with an SVG wire tile
graphics: finish work on high-color forest
graphics: use an SVG-based police station
graphics: make SVG road tiles
TileConstants: implement isCombustible()
TileConstants: replace BLBN, BLBNCN, BNCN constants with definitions

This will make it easier to find and replace uses of BULLBIT, BURNBIT, and
CONDBIT.
tiles.rc: annotate tiles with "noburn" if they are not combustible
tiles.rc: move BURNBIT knowledge from source code to tiles.rc
tiles.rc: annotate conductive tiles
tiles.rc: move CONDBIT knowledge from source code to tiles.rc
tiles.rc: allow inheriting of properties from another tile

To inherit, just put the name of tile you want to inherit from in parenthesis
at the beginning of the specification.
E.g.

road-1-1     (road) roads/1-1 (road-north)(road-south)

will define a tile named "road-1-1", which first starts with all the images
and properties of tile "road" and then layers on top of that the 'road/1-1'
image, and then sets the 'road-north' and 'road-south' flags.
tiles.rc: use inheritance to define roads with traffic