new vips version, 7.40.5

[jcupitt at gmail.com]

There's a new vips up, 7.40.5, with two useful openslide improvements.
Sources plus Window binaries here:

http://www.vips.ecs.soton.ac.uk/supported/7.40/

I've put in an update for homebrew as well, it should appear soon.

1. --autocrop option

openslideload now has a thing to crop the image to just the slide
area. Use it like this:

    vips dzsave CMU-1.svs[autocrop] mydir

And it'll make a deepzoom pyramid of just the slide area.

2. fixed associated image read

There was a long-standing mishap in the thing to read the associated
images from the slide: colour channels would be swapped (or even
missing). This is now fixed.

John