oops
disable sequential skipahead

this was causing some problems on very-many-core systems

still need to update operation flags and disable seq mode for
insert/extract/etc.
remove support for seq mode read for extract etc.

see comment in sequential

We used to not stall if the read position was zero, ie. if the first
request was for a line some way down the image, and assume this was
extract or somesuch. But this could sometimes break on busy, many-core
systems.

Think of a better way to support eg.  extract safely in sequential mode.
Merge remote-tracking branch 'origin/7.38'
Merge remote-tracking branch 'origin/7.38'
notes
backport the webp load from buffer fix
Merge remote-tracking branch 'origin/7.38'
sync
note ottob bug
back out the complex linear stuff

needs cooking for a while longer in a branch
vipsthumbnail copies metadata to embedded thumb

when working from the embedded thumbnail, vipsthumbnail now copies the
metadata from the main image onto the thumbnail

see

https://github.com/jcupitt/libvips/issues/109

thanks ottob
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