tiles.rc: annotate tiles that can be bulldozed

these tiles can be bulldozed clean using the bulldozer
tiles.rc: eliminate BULLBIT and use annotation instead
tiles.rc: synthesize BULLBIT when saving file

this allows files to correctly load in older versions of Micropolis
tiles.rc: re-allow bulldozing dead zones

That is, we allow tiles that belong to zones whose center tile has been destroyed
(e.g. from a fire) to be individually bulldozed. This used to be allowed by setting
the BULLBIT on those tiles when the center tile is destroyed (see killZone() method)
but now that we no longer use BULLBIT, we need to test whether the center tile still
exists when deciding if a tile can be bulldozed.
api: getTile() now returns masked tile number

use getTileRaw() to get the raw tile number
api: use masked tile number for tile-check methods
api: TileConstants- never accept "raw" tile numbers
cleanup: no longer need to apply LOMASK
cleanup: use getTile() instead of direct access where possible
update TODO file
graphics: express coal powerplant smoke as overlay images