jxl: indicate when all frames have been read

and return correct loop count

src/imageformats/jxl_p.h

@@ -57,6 +57,7 @@ private:
         ParseJpegXLNotParsed = 0,
         ParseJpegXLSuccess = 1,
         ParseJpegXLBasicInfoParsed = 2,
+        ParseJpegXLFinished = 3,
     };
 
     ParseJpegXLState m_parseState;