ImVector: Added index_from_pointer() helper.

imgui.h

@@ -1271,6 +1271,7 @@ public:
     inline iterator     erase_unsorted(const_iterator it)               { IM_ASSERT(it >= Data && it < Data+Size);  const ptrdiff_t off = it - Data; if (it < Data+Size-1) memcpy(Data + off, Data + Size - 1, sizeof(value_type)); Size--; return Data + off; }
     inline iterator     insert(const_iterator it, const value_type& v)  { IM_ASSERT(it >= Data && it <= Data+Size); const ptrdiff_t off = it - Data; if (Size == Capacity) reserve(_grow_capacity(Size + 1)); if (off < (int)Size) memmove(Data + off + 1, Data + off, ((size_t)Size - (size_t)off) * sizeof(value_type)); memcpy(&Data[off], &v, sizeof(v)); Size++; return Data + off; }
     inline bool         contains(const value_type& v) const             { const T* data = Data;  const T* data_end = Data + Size; while (data < data_end) if (*data++ == v) return true; return false; }
+    inline int          index_from_pointer(const_iterator it) const     { IM_ASSERT(it >= Data && it <= Data+Size); const ptrdiff_t off = it - Data; return (int)off; }
 };
 
 // Helper: IM_NEW(), IM_PLACEMENT_NEW(), IM_DELETE() macros to call MemAlloc + Placement New, Placement Delete + MemFree