does not work as expected for Viewport and Blender Internal.
As @sergey noticed, this was caused by rBfbf8595, which disabled
rendering for wireframe objects in the viewport. However, the same
function `is_object_hidden` is also used for skipping dupli objects then
which may themselves be renderable.
Now use two variants:
* is_object_restricted is the previous test for general viewport/render visibility (lets wireframe duplicators pass)
* is_object_hidden includes the wireframe check for actual render instances
b2c2edf6ae