note on tiled tiff read

I had a quick go at making tiffload shut down early for tiled tiff read,
but it's not really possible because tiled read is not guaranteed to be
ordered.

Add a note for future reference.

libvips/foreign/tiff2vips.c

@@ -1630,6 +1630,11 @@ rtiff_fill_region( VipsRegion *out,
 
 	VIPS_GATE_STOP( "rtiff_fill_region: work" ); 
 
+	/* We can't shut down the input file early for tile read, even if we
+	 * know load is in sequential mode, since we are not inside a
+	 * vips_sequential() and requests are not guarateed to be in order.
+	 */
+
 	return( 0 );
 }