raise the PNG text chunk limit

Raise the PNG text chunk limit to 50, since it seems some real images
have a text chunk per line of exif data.

See libvips#2433

Merge branch '8.11' of github.com:libvips/libvips into 8.11

not jp2k pyr fix in changelog

add another test for tiff ifd pyramids

test loading from a lower layer

Merge branch 'master' into 8.11

Merge branch 'master' into 8.11

fix build with doxy disabled

note vector fix in changelog

remove mention of exr save in configure

oops

update for newer pandoc

"smart" extension is no longer supported for docbook