Add wxWindow::WXAdjustFontToOwnPPI()

Avoid calling GeTDPI() in font.WXAdjustToPPI(GetDPI()); invocations in
common code on platforms that don't need any adjustment (i.e. anything
other than MSW).

This fixes wxOSX crashes when GetFont() is called too early during
window creation, but is the right thing to do regardless.

Closes https://github.com/wxWidgets/wxWidgets/pull/2036

Closes #18903.

include/wx/window.h

@@ -964,6 +964,13 @@ public:
         // Get the DPI used by the given window or wxSize(0, 0) if unknown.
     virtual wxSize GetDPI() const;
 
+    // Some ports need to modify the font object when the DPI of the window it
+    // is used with changes, this function can be used to do it.
+    //
+    // Currently it is only used in wxMSW and is not considered to be part of
+    // wxWidgets public API.
+    virtual void WXAdjustFontToOwnPPI(wxFont& WXUNUSED(font)) const { }
+
         // DPI-independent pixels, or DIPs, are pixel values for the standard
         // 96 DPI display, they are scaled to take the current resolution into
         // account (i.e. multiplied by the same factor as returned by