Make cursor implementation uniform

Cursors now report their current row number as the boundary of the
partition. This is used by Frame_scan_cursor to compute aggregate
functions that do not support removal.

sql/item_sum.h

@@ -544,6 +544,7 @@ class Item_sum :public Item_func_or_sum
   virtual bool add()= 0;
   virtual bool setup(THD *thd) { return false; }
 
+  virtual bool supports_removal() const { return false; }
   virtual void remove() { DBUG_ASSERT(0); }
 
   virtual void cleanup();
@@ -771,6 +772,11 @@ class Item_sum_sum :public Item_sum_num,
   Item *get_copy(THD *thd, MEM_ROOT *mem_root)
   { return get_item_copy<Item_sum_sum>(thd, mem_root, this); }
 
+  bool supports_removal() const
+  {
+    return true;
+  }
+
 private:
   void add_helper(bool perform_removal);
   ulonglong count;
@@ -829,6 +835,11 @@ class Item_sum_count :public Item_sum_int
   Item *copy_or_same(THD* thd);
   Item *get_copy(THD *thd, MEM_ROOT *mem_root)
   { return get_item_copy<Item_sum_count>(thd, mem_root, this); }
+
+  bool supports_removal() const
+  {
+    return true;
+  }
 };
 
 
@@ -878,6 +889,11 @@ class Item_sum_avg :public Item_sum_sum
   }
   Item *get_copy(THD *thd, MEM_ROOT *mem_root)
   { return get_item_copy<Item_sum_avg>(thd, mem_root, this); }
+
+  bool supports_removal() const
+  {
+    return true;
+  }
 };
 
 
@@ -1089,6 +1105,11 @@ class Item_sum_bit :public Item_sum_int
     DBUG_ASSERT(0);
   }
 
+  bool supports_removal() const
+  {
+    return true;
+  }
+
 protected:
   static const int NUM_BIT_COUNTERS= 64;
   ulonglong reset_bits,bits;