Allow constructing wxGTK wxTextMeasure with NULL font.

The font is explicitly documented as being possibly NULL in the base class and
wxMSW handles NULL font just fine, so also handle it in the GTK version.

See #14706.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72844 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/private/textmeasure.h

@@ -23,7 +23,8 @@ class wxTextMeasureBase
 {
 public:
     // The first ctor argument must be non-NULL, i.e. each object of this class
-    // is associated with either a valid wxDC or a valid wxWindow.
+    // is associated with either a valid wxDC or a valid wxWindow. The font can
+    // be NULL to use the current DC/window font or can be specified explicitly.
     wxTextMeasureBase(const wxDC *dc, const wxFont *theFont);
     wxTextMeasureBase(const wxWindow *win, const wxFont *theFont);
 
@@ -123,6 +124,10 @@ protected:
                            wxCoord *descent = NULL,
                            wxCoord *externalLeading = NULL);
 
+    // Return a valid font: if one was given to us in the ctor, use this one,
+    // otherwise use the current font of the associated wxDC or wxWindow.
+    wxFont GetFont() const;
+
 
     // Exactly one of m_dc and m_win is non-NULL for any given object of this
     // class.