(SNegato2D): take the slide position into account

What does this MR do?

• Take into account the slice position to have the right picking information.
• Retrieve the viewport size in the event method for SNegato2D.

How to test it?

• Launch OgreViewer.
• Open any image.
• Go to the negato view.
• Nothing must have visually changed.

Associated Issues/Merge Requests

• Issues
  • Closes #424 (closed)