Use `thumbnail` when possible

[jcupitt]

Hello, I just found this very nice project.

libvips has a special thumbnail operation which can load and resize an image in one step. Because the two things happen together, it can exploit a lot of tricks with the various load libraries, often giving a large speedup.

For example, here's roughly what intervention-image-vips-driver does now:

$ vipsheader nina.jpg
nina.jpg: 6048x4032 uchar, 3 bands, srgb, jpegload
$ /usr/bin/time -f %M:%e vips resize nina.jpg x.jpg 0.05
103936:0.17

104mb and 0.17s to size down to 302 pixels across. If you use thumbnail instead:

$ /usr/bin/time -f %M:%e vips thumbnail nina.jpg x.jpg 302
44152:0.06

44mb and 0.06s -- half the memory use and almost 3x faster. In fact it's faster still, since in both cases you're paying the same fixed startup cost for libvips:

$ /usr/bin/time -f %M:%e vips
...
31232:0.02

So the real difference is 0.15s vs 0.04, almost 4x faster.

Moreover, thumbnail knows about things like alpha and will premultiply if necessary, so you get better quality too.

I realise load+save in one operation does not fit so well with the architecture you have to work with and might need some thought :(

I noticed some other small things, for example:

https://github.com/osiemsiedem/intervention-image-vips-driver/blob/master/src/Commands/BrightnessCommand.php#L25-L33

You could make this faster by putting the alpha into the brightness array. Something like:

    // what about mono or cmyk images? is intervention always rgb? it might be better
    // to use $core->bands to get the number of image bands rather than assuming three
    $brighten = [$level, $level, $level];
    if ($core->hasAlpha()) {
        $brighten[] = 1;
    }
    
    $core = $core->add($brighten); 

Now there's no need to add and remove the alpha.