fix jxload restart

if the loader was restarted (eg by vipsdisp) it could fail to fetch pixels

libvips/foreign/jxlload.c

@@ -2,6 +2,8 @@
  *
  * 18/3/20
  * 	- from heifload.c
+ * 1/10/21
+ * 	- reset read point for _load
  */
 
 /*
@@ -627,6 +629,17 @@ vips_foreign_load_jxl_load( VipsForeignLoad *load )
 	if( vips_foreign_load_jxl_set_header( jxl, t[0] ) ) 
 		return( -1 );
 
+	/* We have to reset the reader ... we can't reply onb the read point
+	 * being left just after the header.
+	 */
+	if( vips_source_rewind( jxl->source ) )
+                return( -1 );
+
+	if( vips_foreign_load_jxl_fill_input( jxl, 0 ) )
+		return( -1 );
+	JxlDecoderSetInput( jxl->decoder, 
+		jxl->input_buffer, jxl->bytes_in_buffer );
+
 	/* Read to the end of the image.
 	 */
 	do {