engine bugfix: when downgrading full res to little houses, preserve pwrbit

This fixes the bug where a powered RES zone will temporarily flash
"no power" when it downgrades from full to partial.

git-svn-id: http://micropolis.googlecode.com/svn/trunk@780 d9718cc8-9f43-0410-858b-315f434eb58c
TileConstants: move tile number comparisons to central location (road numbers)

git-svn-id: http://micropolis.googlecode.com/svn/trunk@781 d9718cc8-9f43-0410-858b-315f434eb58c
TileConstants: continue moving tile number logic to central location (rails and wires)

git-svn-id: http://micropolis.googlecode.com/svn/trunk@782 d9718cc8-9f43-0410-858b-315f434eb58c