shrink-on-load for openslide images

[jcupitt]

The shrink-on-load logic in thumbnail does not have a path for openslide images.

We'd need to:

• open the base image
• read openslide.level-count to get the number of layers
• starting from level_count - 1 (lowest res), step up the layers until we find one larger than the target (check openslide.level[level].width and .height)
• perhaps should be larger by at least x2? though openslide levels are very spread out, so this could be very expensive
• open that layer as the source image

See libvips/pyvips#72

[renexu]

@jcupitt have you scheduled this enhancement in a future release? when will it be?
Thank you.

[jcupitt]

Yes, it's in 8.8, due in the next month or two. You can build git master and get it now if you want.

[renexu]

Thank you @jcupitt