emit "finish" at the end of targetcustom write

In 8.13 we deprecated target_finish() (it did not have an error return)
to target_end() (now has an error return).

This PR makes target_end() also emit the "finish" signal for
compatibility with the previous API. ruby-vips, for example, looks for
on_finish() to close files.

See https://github.com/libvips/ruby-vips/issues/351

Thanks lucaskanashiro

ChangeLog

@@ -1,5 +1,6 @@
 9/11/22 started 8.13.4
 - missing include in mosaic_fuzzer [ServOKio]
+- emit "finish" at the end of targetcustom write [lucaskanashiro]
 
 11/10/22 started 8.13.3
 - improve rules for 16-bit heifsave [johntrunc]

libvips/iofuncs/targetcustom.c

@@ -161,6 +161,11 @@ vips_target_custom_end_real( VipsTarget *target )
 
 	VIPS_DEBUG_MSG( "vips_target_custom_end_real:\n" );
 
+	/* For compatibility with oklder libvipses, we have to emit "finish"
+	 * as well. ruby-vips relies on this.
+	 */
+	g_signal_emit( target, vips_target_custom_signals[SIG_FINISH], 0 );
+
 	/* Return value if no attached handler.
 	 */
 	result = 0;