Fix a quadratic behavior in the BiDi algorithm

Reset the lastETPosition after we changed DirET to DirEN,
to avoid iterating over the same set of characters many times.

Change-Id: Ib4113d0ba87ad70fc6bb386632eb094f943c080d
Reviewed-by: Robert Loehning <robert.loehning@qt.io>

src/gui/text/qtextengine.cpp

@@ -712,9 +712,8 @@ struct QBidiAlgorithm {
                         analysis[pos].bidiDirection = QChar::DirEN;
                         ++it;
                     }
-                } else {
-                    lastETPosition.clear();
                 }
+                lastETPosition.clear();
             }
             last = current;
             lastPos = pos;