don't use thumnail_image

[jcupitt]

Hi, I just stumbled upon this nice project.

I noticed it's using thumbnail_image:

https://github.com/zRedShift/thumbnailer/blob/master/vips.c#L59

This is very slow --- it's much better to use thumbnail or thumbnail_buffer, if possible. For example:

$ vipsheader theo.jpg
theo.jpg: 6048x4032 uchar, 3 bands, srgb, jpegload
$ time vips thumbnail theo.jpg x.jpg 256

real	0m0.102s
user	0m0.084s
sys	0m0.034s
$ time vips thumbnail_image theo.jpg x.jpg 256

real	0m1.697s
user	0m3.819s
sys	0m0.109s

So for a 6k x 4k pixel JPEG, thumbnail is around 15x faster.

Combining open and resize in one operation lets libvips exploit a range of tricks with resize options in the load library. You'll get better quality for things like PDF as well.