(ExImageMix): image voxel information is not correct"

What does this MR do?

(Briefly describe what this MR is about)

How to test it?

• Launch ExImageMix
• load two images (background and front)
• click on the background image The displayed information should now correspond.

Some results

Additional tests to run

(If you need some specific test e.g Windows compilation...)

• Build on Linux

Associated Issues/Merge Requests

• Issues
  • Fixes Sight/sight#155