ImPool: Added Reserve() helper function.

imgui_internal.h

@@ -241,6 +241,7 @@ struct IMGUI_API ImPool
     T*          Add()                               { int idx = FreeIdx; if (idx == Data.Size) { Data.resize(Data.Size + 1); FreeIdx++; } else { FreeIdx = *(int*)&Data[idx]; } IM_PLACEMENT_NEW(&Data[idx]) T(); return &Data[idx]; }
     void        Remove(ImGuiID key, const T* p)     { Remove(key, GetIndex(p)); }
     void        Remove(ImGuiID key, int idx)        { Data[idx].~T(); *(int*)&Data[idx] = FreeIdx; FreeIdx = idx; Map.SetInt(key, -1); }
+    void        Reserve(int capacity)               { Data.reserve(capacity); Map.Data.reserve(capacity); }
 };
 typedef int ImPoolIdx;