Make FMVPlayer in TD use a completely black background

Fixes #6211.
Make FMVPlayer in RA use a completely black background
5936 Advanced Power Plant have a different layout in the original
Merge pull request #6230 from havok13888/AdvancedPowePlant

#5936 Advanced Power Plant have a different layout in the original
Merge pull request #6220 from obrakmann/fix6211

Make background of FMVPlayers in TD and RA black
avoid NullReferenceException in updatedRegion

fixes #6166
Merge pull request #6226 from Mailaender/shroud-updatedregion-nre

Fixed a crash when there are no actors to reveal shroud on the map.
Added `int subCell = -1` to IMove.MoveIntoWorld & IPositionable.SetPosition(Actor self, CPos cell)
Refactor MobileInfo.CanEnterCell
Fixed UnloadCargo stacking using new subcell API exposure
Refactored [sub-]cell availability logic in IPositionable & ActorMap

Moved the logic from IPositionable.CanEnterCell & integrated sub-cell selection.
Added IPositionable.IsMovingFrom(CPos location, int subCell = -1) - to detect transient actors
Renamed IPositionable.{GetDesiredSubcell => GetAvailableSubcell} - since it checks for available sub-cells
Reduced IPositionable.CanEnterCell to one method that usually uses IPositionable.GetAvailableSubcell
Added actor checking to ActorMap.{HasFreeSubCell, FreeSubCell, AnyUnitsAt} - used by [sub-]cell availability logic
Folded MobileInfo.CanEnterCell overloads into one.