version bump for 7.38

and fix a few dist problems
bump soname

oops, thanks benjamin
pack Radiance images to and from 0-1

so this now works:

$ vips copy uffizi_probe.hdr x.jpg
$ eog x.jpg
$ vips copy x.jpg x.hdr
$ vips copy x.hdr x2.jpg
$ eog x2.jpg