Mesh opacity is not alpha opacity but additive should be alpha blending

[axtimwalde]

seems to work correctly for orthoslices if that helps, probably a weird material feature, or lack of depth sorting? Also, we should add an option to adjust the size of for adjacent mesh cubes. We need this for covering up transitions between low and high resolution mesh cubes but it becomes visible with transparent meshes and we probably want to set it to 0 then.