clean up the vipsthumbnail fix

nicer way to copy the metadata
fix to vips_grey() --uchar
Merge remote-tracking branch 'origin/7.38'

Conflicts:
	ChangeLog
	configure.ac
Merge branch 'master' of github.com:jcupitt/libvips
add basic auto-rotate support to vipsthumbnail
argh revert complex linear again

how did it get tangled up
more vipsthumbnail --rotate work

now needs some changes to jpegsave
move orientation tag from vipsthumbnail output
remove the embedded thumbnail reader

remove the embedded thumbnail reader, embedded thumbnails are too unlike
the main image wrt. rotation / colour / etc.
handle other exif orientation tag values

though not the mirroring ones
small fixes
vipsthumbnail fix

fix to vipsthumbnail ... --crop could fail for very non-square images,
thanks Alessandro
add vips_sum()

sum an array of images
fix vipsthumbnail crop with very non-square images

thanks Alessandro
changelog update too
fix leak in recomb.c
Merge branch 'master' into hough
stuff
README updates
oops should not have gtk-doc.make in the repo