(svn r11399) -Feature(ette): transparency settings can now be saved and thus remembered.

-Codechange: Wrap all transparency settings in accessors, hiding the implementation
-Change: Clicking "transparent building" menu will toggle Houses And Trees only.  The other options can be used in the transparency gui.
Initial patch by Smatz (FS#1349), with some rework by BigBB and your humble servant.

src/openttd.cpp

@@ -321,9 +321,6 @@ static void LoadIntroGame()
 {
 	_game_mode = GM_MENU;
 
-	/* Clear transparency options */
-	_transparent_opt = 0;
-
 	_opt_ptr = &_opt_newgame;
 	ResetGRFConfig(false);