Add ability to render checkbox in undeterminate state, fixes #12290: wxRendererGTK does not support wx.CONTROL_UNDETERMINED for checkboxes

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65139 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/gtk/renderer.cpp

@@ -469,12 +469,21 @@ wxRendererGTK::DrawCheckBox(wxWindow* win,
     else
         state = GTK_STATE_NORMAL;
 
+    GtkShadowType shadow_type;
+
+    if ( flags & wxCONTROL_UNDETERMINED )
+        shadow_type = GTK_SHADOW_ETCHED_IN;
+    else if ( flags & wxCONTROL_CHECKED )
+        shadow_type = GTK_SHADOW_IN;
+    else
+        shadow_type = GTK_SHADOW_OUT;
+
     gtk_paint_check
     (
         button->style,
         gdk_window,
         state,
-        flags & wxCONTROL_CHECKED ? GTK_SHADOW_IN : GTK_SHADOW_OUT,
+        shadow_type,
         NULL,
         button,
         "cellcheck",