link from a piece table cursor object back to the containing piece table

src/core/PieceTable.cc

@@ -42,6 +42,7 @@ std::shared_ptr<PieceTable::Cursor> PieceTable::add_cursor()
 {
     auto cursor = std::make_shared<Cursor>();
 
+    cursor->piece_table = this;
     cursor->piece = start_piece->next;
     cursor->line_number = 0u;
     cursor->offset = 0u;

src/core/PieceTable.h

@@ -49,6 +49,9 @@ public:
 
     struct Cursor
     {
+        /** The piece table the cursor belongs to. */
+        PieceTable * piece_table;
+
         /** The piece. */
         Piece * piece;