Use static buffer for fake JPEG EOI marker as src->buf might not be allocated

Discovered whilst investigating lovell/sharp#6
fix webp load from buffer

was broken horribly, see

https://github.com/lovell/libvips/commit/fa8b7e66ca70d21602dfe2aa0210371952cd2b51
jpeg load from buffer shouldn't modify buffer

fill_input_buffer() could write to the input buffer

see: https://github.com/jcupitt/libvips/pull/107

thanks Lovell
Merge pull request #107 from lovell/master

Use static buffer for fake JPEG EOI marker as src->buf might not be allocated
Merge branch '7.38'

Conflicts:
	ChangeLog
	configure.ac
	libvips/foreign/jpeg2vips.c
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
Merge remote-tracking branch 'origin/master' into complex-constants
paste complex linear back again