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: 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: fix bug in SVG->PNG logic
graphics: use an SVG-based police station
graphics: make SVG road tiles
MakeTiles: fix bug where unable to make generated image in subdirectory