If the MxN Display is used it might happen that a (reference and working image ) node on which a label should be drawn is globally invisible (global property) but visible in a render window (renderer-specific property). This leads to the Segmentation view displaying a warning and disabling the segmentation tools.

In order to avoid this and to allow users to draw a mask in such a scenario, the warning should not be shown and the segmentation tool / tool selection boxes should not be disabled.

To reproduce:

1. open MITK, load an image
2. close the Standard Display, open the MxN Display, create two render windows
3. open the Segmentation view, create a new segmentation node
4. decouple / unlock one render window (lock symbol)
5. hide / deselect the reference / working image node in the still locked render window
6. see how the required nodes are visible in the unlocked render window but the segmentation tools are disabled