GetMouseDragDelta(): also returns the delta on the mouse button released frame. Verify that mouse positions are valid otherwise returns zero. Removed obsolete comment. Tweaked demo. (#2419)

imgui.h

@@ -662,7 +662,7 @@ namespace ImGui
     IMGUI_API bool          IsMousePosValid(const ImVec2* mouse_pos = NULL);                    // by convention we use (-FLT_MAX,-FLT_MAX) to denote that there is no mouse
     IMGUI_API ImVec2        GetMousePos();                                                      // shortcut to ImGui::GetIO().MousePos provided by user, to be consistent with other calls
     IMGUI_API ImVec2        GetMousePosOnOpeningCurrentPopup();                                 // retrieve backup of mouse position at the time of opening popup we have BeginPopup() into
-    IMGUI_API ImVec2        GetMouseDragDelta(int button = 0, float lock_threshold = -1.0f);    // return the delta from the initial clicking position. This is locked and return 0.0f until the mouse moves past a distance threshold at least once. If lock_threshold < -1.0f uses io.MouseDraggingThreshold
+    IMGUI_API ImVec2        GetMouseDragDelta(int button = 0, float lock_threshold = -1.0f);    // return the delta from the initial clicking position while the mouse button is pressed or was just released. This is locked and return 0.0f until the mouse moves past a distance threshold at least once. If lock_threshold < -1.0f uses io.MouseDraggingThreshold.
     IMGUI_API void          ResetMouseDragDelta(int button = 0);                                //
     IMGUI_API ImGuiMouseCursor GetMouseCursor();                                                // get desired cursor type, reset in ImGui::NewFrame(), this is updated during the frame. valid before Render(). If you use software rendering by setting io.MouseDrawCursor ImGui will render those for you
     IMGUI_API void          SetMouseCursor(ImGuiMouseCursor type);                              // set desired cursor type