message: dx: when filling an HWND target, use the actual background color
  There is no guarantee that the HWND's backing color matches our expected
  backing color.
  This repairs the gutter in the WPF terminal.
diff: @@ -1177,7 +1177,12 @@ CATCH_RETURN()
  [[nodiscard]] HRESULT DxEngine::PaintBackground() noexcept
  try
  {
  - D2D1_COLOR_F nothing = { 0 };
  + D2D1_COLOR_F nothing{ 0 };
  + if (_chainMode == SwapChainMode::ForHwnd)
  + {
  + // When we're drawing over an HWND target, we need to fully paint the background color.
  + nothing = _backgroundColor;
  + }
  // If the entire thing is invalid, just use one big clear operation.
  if (_invalidMap.all())
  