Improve handling of non default DPI in wxHtmlWindow deawing code

This replaces the fix of f4dcac9f44 (Return wxFont adjusted to DPI in
wxHtmlWinParser::CreateCurrentFont(), 2020-11-25) adjusting the font
explicitly under MSW with a better fix, associating the correct window
(and hence DPI) with the device context used in OnPaint().

See #18564.

src/html/htmlwin.cpp

@@ -1116,6 +1116,7 @@ void wxHtmlWindow::OnPaint(wxPaintEvent& WXUNUSED(event))
         dc = &dcm;
     }
 
+    dc->GetImpl()->SetWindow(this);
     PrepareDC(*dc);
 
     // Erase the background: for compatibility, we must generate the event to