add OpenSlide properties to image metadata
add OpenSlide properties to image metadata

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