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)