set reduced-resolution flag when writing TIFF pyramid layers
set reduced-resolution flag when writing TIFF pyramid layers

TIFF pyramid layers other than the full-resolution image should be tagged
with a TIFFTAG_SUBFILETYPE of FILETYPE_REDUCEDIMAGE.

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