QJniArray: implement offset dereference operator

The last requirement for making the iterator random-access, so
change category type.

Based on review comments.

Task-number: QTBUG-126150
Change-Id: I617f38f92d0f9279781e62ea3ab1929dbf6a07cd
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
(cherry picked from commit e4b2d7607c0243f2a7ca3f38f59e8532c543fcc7)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/kernel/qjniarray.h

@@ -37,12 +37,18 @@ struct QJniArrayIterator
     using pointer = T *;
     using reference = T; // difference to container requirements
     using const_reference = reference;
-    using iterator_category = std::bidirectional_iterator_tag;
+    using iterator_category = std::random_access_iterator_tag;
 
     const_reference operator*() const
     {
         return m_array->at(m_index);
     }
+
+    const_reference operator[](difference_type n) const
+    {
+        return m_array->at(m_index + n);
+    }
+
     friend QJniArrayIterator &operator++(QJniArrayIterator &that) noexcept
     {
         ++that.m_index;

src/corelib/kernel/qjniarray.qdoc

@@ -328,7 +328,7 @@
 /*!
     \typedef QJniArray::const_iterator
 
-    A bi-directional, const iterator for QJniArray.
+    A random-access, const iterator for QJniArray.
 */
 
 /*!

tests/auto/corelib/kernel/qjniarray/tst_qjniarray.cpp

@@ -251,6 +251,9 @@ void tst_QJniArray::operators()
         QCOMPARE(*it, 'd');
         it = array.size() - it;
         QCOMPARE(*it, 'c');
+
+        QCOMPARE(it[1], 'd');
+        QCOMPARE(it[-1], 'b');
     }
     {
         auto it = array.rbegin();