This patch will show the face orientation overlay when using the
workbench in scenes with both solid and transparent objects.
It used to be that the depth buffer didn't contain the transparent
object. This patch renders the transparent objects in the depth buffer,
but only when it is needed for an active overlay.
When updating the depth to have all objects the overlay wires could get
occluded. In the future we would like to do proper front and back wire
overlay drawing. Due to this we kept this patch as simple as possible.