Fix where interpolate_pixels server config is read from

[Tom-TBT]

I realized that iviewer was not able to read the interpolate_pixels configuration.

As it was read, interpolate_pixels should be part of the client settings
→ omero.client.interpolate_pixels

But if I print the server_settings after setting that property, it only appears under ["viewer"]["interpolate_pixels"] and has the default value True

However if I change the configuration to:
→ omero.client.viewer.interpolate_pixels (as listed in the config glossary)
then it reads the configuration properly.

So unless there was a way to read the configuration property, this is the fix to read the viewer interpolate_pixels from the server settings.