BattlescapeGame: stop calcFoV in popState()

Map: adjust pathPreview colors
Pathfinding: prevent Mc'd aLiens who cant walk from ALT-forcewalk
	- try to tidy/fixup _kneelCheck/gravLift chicanery
ScannerState: no LMB to exit, RMB only
TileEngine: fix smokeBlockage
UnitWalkBState: fixup kneeled/gravLifts ambiguity