starting maplut
null-terminate exif strings

libexif strings are not null-terminated, add a \0 before adding to the
vips image

thanks Mike
Merge remote-tracking branch 'origin/7.34'
fix a used-before-set valgrind complaint
vipsthumbnail rewrite
add multiscan jpg write?
fix some typos
add a "const" to get_string
oops another const
fix up -DVIPS_DEBUG build
more maplut hacking
Fix three minor memleaks
sync
make VipsArea leakcheck a run-time option

was compile-time
more robust error handling for libpng
move im_maplut() to a class
cache should be threaded
redo falsecolour, gammacorrect as classes
deprecate im_hist()

a useless combination of find and plot
TODO update