Fix bug which caused the diagonal line in volume rendering areas.
Fix bug which caused the diagonal line in volume rendering areas.

This was caused by discarding certain fragments on diagonal of the
whole image. Somehow there is some kind of multi-sampling active
and by just using a portion of the samples the result had a wrong
color (including a wrong alpha value).

Christof/voly-labeller,Christof/voly-labeller,Christof/voly-labeller,Christof/voly-labeller