Smart crop support

[h2non]

https://github.com/muesli/smartcrop

[jcupitt]

Alternative smart crop in libvips:

https://gist.github.com/jcupitt/c2792a9b22e4e2544147

It's a slightly different algorithm. Rather than attempting to find the interesting part of an image, all it does is trim the most boring parts (found by estimating image entropy) off the longest axis until the image is square.

The idea is that the most common use for smartcrop is to square up images so that you can display a large number of images with different aspect ratios in a nice grid. This is a much simpler problem than trying to find the centre of interest in a picture.

[Dynom]

As of libvips version 8.5.0, there is a "smart crop" feature available. It's based on Sharp's smartcrop, which I think is the same as linked earlier here in the Issue.

See also: https://github.com/jcupitt/libvips/blob/c79f64ab7def94b23ca65abf46c4ca9e97e3c5db/libvips/conversion/smartcrop.c

[greut]

should be closed by #145

[h2non]

Smartcrop feature has been shipped in v1.0.8. Thank you @greut!