UPSTREAM-REJECTED: src/gui/imagewidget.cpp: use NEAREST filter for image scaling

When zooming in to more than one on-screen pixel per data pixel,
this gives us nice discrete zoomed-in pixels, rather than soft
blending between them all.

*** Rejected by upstream since he's in a case where the underlying image size
is often much much bigger than the window. I like my patch so I'm keeping it.
***