Avoid inadvertently destroying a shared PangoContext on GTK2

wxMemoryDC and wxScreenDC dtors were freeing a context they didn't own, potentially
causing a crash. Fixed by having the base wxWindowDC dtor free the context.
See #18566

src/gtk/dataview.cpp

@@ -2629,6 +2629,7 @@ public:
         m_window = window;
 
         m_context = window->GTKGetPangoDefaultContext();
+        g_object_ref(m_context);
         m_layout = pango_layout_new( m_context );
         m_fontdesc = pango_font_description_copy(gtk_widget_get_style(widget)->font_desc);
 

src/gtk/dcclient.cpp

@@ -304,6 +304,7 @@ wxWindowDCImpl::wxWindowDCImpl( wxDC *owner, wxWindow *window ) :
     }
 
     m_context = window->GTKGetPangoDefaultContext();
+    g_object_ref(m_context);
     m_layout = pango_layout_new( m_context );
     m_fontdesc = pango_font_description_copy( widget->style->font_desc );
 
@@ -344,6 +345,8 @@ wxWindowDCImpl::~wxWindowDCImpl()
 {
     Destroy();
 
+    if (m_context)
+        g_object_unref(m_context);
     if (m_layout)
         g_object_unref (m_layout);
     if (m_fontdesc)
@@ -1539,6 +1542,10 @@ void wxWindowDCImpl::SetFont( const wxFont &font )
             // at least, and it doesn't hurt to do it.
             if (oldContext != m_context)
             {
+                g_object_ref(m_context);
+                if (oldContext)
+                    g_object_unref(oldContext);
+
                 if (m_layout)
                     g_object_unref (m_layout);
 

src/gtk/dcmemory.cpp

@@ -40,7 +40,6 @@ wxMemoryDCImpl::wxMemoryDCImpl( wxMemoryDC *owner, wxDC *WXUNUSED(dc) )
 
 wxMemoryDCImpl::~wxMemoryDCImpl()
 {
-    g_object_unref(m_context);
 }
 
 void wxMemoryDCImpl::Init()

src/gtk/dcscreen.cpp

@@ -50,8 +50,6 @@ void wxScreenDCImpl::Init()
 
 wxScreenDCImpl::~wxScreenDCImpl()
 {
-    g_object_unref(m_context);
-
     gdk_gc_set_subwindow( m_penGC, GDK_CLIP_BY_CHILDREN );
     gdk_gc_set_subwindow( m_brushGC, GDK_CLIP_BY_CHILDREN );
     gdk_gc_set_subwindow( m_textGC, GDK_CLIP_BY_CHILDREN );