small tiff load improvements
small tiff load improvements

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