earlier quit of dzsave on kill

We were only testing "killed" once for each strip of tiles. Check on each
tile as well to make set_kill() faster.

see https://github.com/kleisauke/net-vips/issues/179

ChangeLog

@@ -3,6 +3,7 @@
 - fix 8 bit pallete PNG save [lovell]
 - fix null string in buffer print [pclewis]
 - revise caching of seq mode loaders [jcupitt]
+- reduce latency on dzsave kill [kleisauke]
 
 24/7/22 started 8.13.1
 - fix im7 feature detection in meson

libvips/foreign/dzsave.c

@@ -1828,6 +1828,12 @@ strip_work( VipsThreadState *state, void *a )
 	printf( "strip_work\n" );
 #endif /*DEBUG_VERBOSE*/
 
+        /* killed is checked by sink_disc, but that's only once per strip, and
+         * they can be huge. Check per output tile as well.
+         */
+	if( vips_image_iskilled( save->in ) )
+		return( -1 );
+
 	/* If we are centering we may be outside the real pixels. Skip in 
 	 * this case, and the viewer will display blank.png for us. 
 	 */