fixed cell background overlapping with header

[prabuinet]

This commit fixes the following issue:

#7041

[prabuinet]

code to recreate the issue:

  if (ImGui::BeginTable("Test", 1, ImGuiTableFlags_Resizable | ImGuiTableFlags_NoSavedSettings | ImGuiTableFlags_Borders | ImGuiTableFlags_ScrollY))
  {
      static bool selected[100] = {};

      ImGui::TableSetupScrollFreeze(0, 1);
      ImGui::TableSetupColumn("Test Column", ImGuiTableColumnFlags_None);
      ImGui::TableHeadersRow();
      for (int i = 0; i < 100; i++)
      {
          char label[32];
          sprintf(label, "Item %d", i);
          ImGui::TableNextRow();
          ImGui::TableNextColumn();
          ImGui::TableSetBgColor(ImGuiTableBgTarget_CellBg, ImGui::GetColorU32(ImVec4(0.9f, 0.3, 0.3f, 0.6f)), 0);
          ImGui::Text(label);
      }
      ImGui::EndTable();
  }

[ocornut]

Thank you @prabuinet, this is correct and indeed fixes the issue. Merged now!