Fix nikon d1x madness

Nikon d1x has non-square pixels, so we need to stretch them.
I put this iop after demosaic, after spots, before lens

``FIXME: is this the best position?``
``FIXME: is *scalepixels* is the best name? *pixelstretch* seems like a better name since IOP only scales pixels up in given direction, not down. (only increases dimensions, not decreases)``

Sometimes, on first image load, image is not being stretched, and is being stretched on second load.
No idea what causes it. (was also seen happening for CFA, raw black/white levels(#619), raw crop).

All OpenCL-related things was moved to #668.