Fix font creation using font flags in Cairo wxGraphicsContext

Don't crash trying to use an uninitialized font in
GetPartialTextExtents() later if the font was created using
CreateFont(sizeInPixels, facename, flags) overload.

Closes #18021.

src/generic/graphicc.cpp

@@ -1013,6 +1013,10 @@ wxCairoFontData::wxCairoFontData(wxGraphicsRenderer* renderer,
                                  int flags,
                                  const wxColour& col) :
     wxGraphicsObjectRefData(renderer)
+#ifdef __WXGTK__
+    , m_wxfont(wxFontInfo(wxSize(sizeInPixels, sizeInPixels))
+                .AllFlags(flags).FaceName(facename))
+#endif
 {
     InitColour(col);