fix tiff pyramid save region-shrink

we'd forgotton to connect it up

thanks imgifty

see https://github.com/libvips/libvips/issues/1875

ChangeLog

@@ -3,6 +3,7 @@
 - fix vips7 webp load [barryspearce]
 - fix out of bounds exif read in heifload
 - fix out of bounds read in tiffload
+- fix tiffsave region shrink mode [imgifty]
 
 6/9/20 started 8.10.2
 - update magicksave/load profile handling [kelilevi]

libvips/foreign/vips2tiff.c

@@ -1629,7 +1629,8 @@ layer_strip_shrink( Layer *layer )
 		if( vips_rect_isempty( &target ) )
 			break;
 
-		(void) vips_region_shrink( from, to, &target );
+		(void) vips_region_shrink_method( from, to, &target,
+			layer->wtiff->region_shrink );
 
 		below->write_y += target.height;