Fix for problem with non-finite values in histogram and stddev auto cuts
Fix for problem with non-finite values in histogram and stddev auto cuts

- fixes an issue (#549) where some images with too many NaNs in the crop
  could not calculate the cut levels correctly.  This checks for the
  percentage of usable data in the crop and falls back to the full image
  for calculation if there is less than 50% usable pixels in the crop

naojsoft/ginga,pllim/ginga,ejeschke/ginga,naojsoft/ginga,naojsoft/ginga,ejeschke/ginga,pllim/ginga,pllim/ginga,ejeschke/ginga