fix dzsave iiif dimensions

dzsave in iiif mode could set info.json dimensions off by one

thanks Linden6

see https://github.com/libvips/libvips/issues/1818

ChangeLog

@@ -4,6 +4,7 @@
 - fix tiff thumbnail from buffer and source [vansante]
 - in jpegsave, don't set JFIF resolution if we set EXIF resolution
 - bump minimum libheif version to 1.3 [lovell]
+- dzsave in iiif mode could set info.json dimensions off by one [Linden6]
 
 9/8/20 started 8.10.1
 - fix markdown -> xml conversion in doc generation

libvips/foreign/dzsave.c

@@ -1049,8 +1049,8 @@ write_json( VipsForeignSaveDz *dz )
 	gsf_output_printf( out, 
 		"  \"width\": %d,\n"
 		"  \"height\": %d\n", 
-			dz->layer->image->Xsize,
-			dz->layer->image->Ysize );
+			dz->layer->width,
+			dz->layer->height );
 
 	gsf_output_printf( out, 
 		"}\n" );