vips_unpremultiply() always outputs float. If you save the output of
vipsthumbnail to a format that supports float files, like tiff, it won't
ever get cast back to the source format.
Example:
$ vipsheader Opera-icon-high-res.png
Opera-icon-high-res.png: 3056x3325 uchar, 4 bands, srgb, pngload
$ vipsthumbnail Opera-icon-high-res.png -o x.tif
$ vipsheader x.tif
x.tif: 117x128 float, 4 bands, scrgb, tiffload
This change makes it note and restore BandFmt aropund pre/unpremultiply.
See https://github.com/jcupitt/libvips/issues/447
54d8e4808f