drawing-area: fix crash bug caused by not loading sprite images
drawing-area: fix where sprites are drawn when zoom level is not 16px
drawing-area: scale sprite images with tile resolution
TileSpec: fix bug with parsing of 'noburn'
tile-animation: let tiles.rc determine coal plant's smoke animation
engine: only use PWRBIT flag on zone center tiles
tile animation: use tiles.rc to control termination of coal plant smoke

when the coal plant is killed

Also, this sets up a framework to handle industrial zones in the same way.
tile animation: make industrial zone smoke animations initiated by tiles.rc
tile animation: make nuclear swirl handled same as coal smoke

that is, it starts only when setZonePower() is called for that zone,
and it automatically stops when killZone() is invoked.