MakeTiles: impl support for SVG files

SVG files are converted to PNG files at the appropriate resolution
by calling out to Inkscape.exe (must be installed somewhere)
MakeTiles: when generating intermediate images, use a staging directory
MakeTiles: fix bug in SVG->PNG logic
MakeTiles: when converting an SVG file, specify DPI rather than expected width/height

This way we can handle multi-tile SVG source files (e.g. for the larger zones) correctly.
MakeTiles: allow making generated images of an SVG image in a subdirectory
drawing-area: hold in-memory tile images in an object, rather than static variables

This will allow changing tile sizes on a per-window basis.
drawing-area: allow zooming in to 32px tiles
tiles: transfer ZONEBIT knowledge to tiles.rc

and synthesize ZONEBIT,ANIMBIT when saving (for compatibility)
tiles.rc syntax: allow inheriting of another tile's images/attributes

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.
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: 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