A bunch of level editor fixes in this one.

Fixed bugs:

• Leaving the level editor with the "flatten area" window open could crash the game
• Moving a landscape flatten area in the level editor would not automatically regenerate the landscape mesh
• If no lights were defined for a level (e.g. unfinished maps in mods or similar) the game could crash on entering the level
• The circle around SafeArea buildings was not rendering correctly in the level editor.