Begin: Removed asserts that got in the way of some flags combination. (#1345)

imgui.cpp

@@ -4649,12 +4649,7 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
 
         // Apply minimum/maximum window size constraints and final size
         window->SizeFull = CalcSizeAfterConstraint(window, window->SizeFull);
-        window->Size = window->Collapsed ? window->TitleBarRect().GetSize() : window->SizeFull;
+        window->Size = window->Collapsed && !(flags & ImGuiWindowFlags_ChildWindow) ? window->TitleBarRect().GetSize() : window->SizeFull;
-        if ((flags & ImGuiWindowFlags_ChildWindow) && !(flags & ImGuiWindowFlags_Popup))
-        {
-            IM_ASSERT(window_size_x_set_by_api && window_size_y_set_by_api); // Submitted by BeginChild()
-            window->Size = window->SizeFull;
-        }
 
         // SCROLLBAR STATUS