So. Some themes have objects that seem to be large natural extensions of the landscape. Masks allow maintaining that. Lemme know if it doesn't look good. If it doesn't, can still use for ice/bounce/indestructible. Indestructible bunker object for example.

foreach(dir "Fonts" "Forts" "Graphics" "Locale" "Maps" "Music" "Sounds" "Themes" "Missions" "Names" "misc" "Scripts")
  add_subdirectory(${dir})
endforeach(dir)