Window: Fixed SetNextWindowSizeConstraints() with non-rounded positions making windows drift. (#2067, #2530)

imgui.cpp

@@ -4648,6 +4648,8 @@ static ImVec2 CalcSizeAfterConstraint(ImGuiWindow* window, ImVec2 new_size)
             g.NextWindowData.SizeCallback(&data);
             new_size = data.DesiredSize;
         }
+        new_size.x = ImFloor(new_size.x);
+        new_size.y = ImFloor(new_size.y);
     }
 
     // Minimum size