working quick clip/zoom of xtrans data

dt_iop_clip_and_zoom_demosaic_onethird_size_xtrans() works now,
parallel to dt_iop_clip_and_zoom_demosaic_half_size(). Not most
optimized code.

NOTES:
- The upper limits for px/py and maxi/maxj aren't tested
- Things may get odd if scale is > 2/3, but that shouldn't happen?