in result of some comments in #166130 I have been looking at image scaling  
in scale-funcs.c

It seems the needs of scale down are quite different from the  
interpolation of scaling up so there are certain compromises in using the  
same lin/cubic/lanczos list of options.

currently linear and cubic both execute the same code which just averages  
all source pixels, it is neither linear or cubic.

Also downscaling has different needs depending on whether the scaling is  
near to unity or not.

I think cubic (catmull-rom as per scale up) should give better results. It  
may be necessary to decide a policy for reductions greater than 4:1 to  
avoid ignoring some data points.

After that linear may benefit from a weighted mean rather than just  
lumping all the source pixels together. This may reduce the softening  
currently seen.

Currently pretending to do cubic and just averaging is not too good.

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