fix an obscure dzsave bug
fix an obscure dzsave bug

in some cases dzsave could fail to write all the pyramid layers out

the problem was if a layer was odd-sized and needed rounding up so it
could be shrunk from, and if the tile size was such that the final strip
was sized exactly to fit the last line of pixels that would come into
this layer, the final line of pixels could never be interpolated

thanks to Martin for the report

lovell/libvips,wishabi/libvips,jcupitt/libvips,jcupitt/libvips,leiyangyou/libvips,jcupitt/libvips,jcupitt/libvips,leiyangyou/libvips,mcanthony/libvips,wishabi/libvips,leiyangyou/libvips,wishabi/libvips,mcanthony/libvips,lovell/libvips,wishabi/libvips,leiyangyou/libvips,mcanthony/libvips,mcanthony/libvips,lovell/libvips,lovell/libvips