visibility_in, visibility_out
visibility_out controls whether the layer, when current, attempts 
to see the other layers: 
all - try to see the other layers; 
partial - try to see the other layers in a blurred way, to distinguish 
them from the current layer; 
none - ignore the other layers.
visibility_in controls whether the layer autorizes other layers,
when current, to look inside: 
all - the layer 
can be seen; 
partial: the layer can be seen in a 
blurred way, to distinguish it from the current layer;
none - the layer cannot be seen from the outside. 
visibility_in (a privacy condition) has always
priority over 
visibility_out (a curiosity condition).
By default layers are completely transparent, to the inside and 
to the outside. 
Example: <layer ... visibility_in="all" visibility_out="all"/> (default)
(defined in GAMGI_MESA_TRANSPARENT)
Allowed values: all, partial, none (optional)