#98 Nearest neighbor interpolation only working for main image

[adhusch]

Tested with ITKSnap 3.6.

Steps to reproduce:
a) load a main image
b) add an additional image
c) set preferences / slices views / interpolation mode: "Nearest neighbor"

Result: Main image is draw using nearest neighbor interpolation, however all additional images still use linear interpolation.

Cheers
Andy