[glade] Fix use of GTK+ style context in GladeDesignLayout.

[Tristan Van Berkom <tvb src gnome org>]

commit ac55fa78566145ac44d48ba88ec1351db5b4a99d
Author: Arnaud Rebillout <arnaud preev io>
Date:   Tue Jun 13 15:03:02 2017 +0700

    Fix use of GTK+ style context in GladeDesignLayout.
    
    It seems like modifying the style context in the 'draw' handler is not
    recommended, and we need to save/restore the context.
    
    Otherwise, for some widgets (GtkButton, GtkComboBox), the
    GladeDesignLayout gets trapped in draw-damage loop.
    
    See 0c076cc8828cd80f1f156a08569199675bf35165 for reference.

 gladeui/glade-design-layout.c |    7 ++++---
 1 files changed, 4 insertions(+), 3 deletions(-)
---
diff --git a/gladeui/glade-design-layout.c b/gladeui/glade-design-layout.c
index b5970ce..17e9a60 100644
--- a/gladeui/glade-design-layout.c
+++ b/gladeui/glade-design-layout.c
@@ -1033,13 +1033,13 @@ draw_frame (GtkWidget *widget, cairo_t *cr, gboolean selected,
   if (priv->widget_name)
     {
       GdkRectangle *rect = &priv->south_east;
-
+      gtk_style_context_save (context);
       gtk_style_context_add_class (context, "handle");
       gtk_render_background (context, cr, rect->x, rect->y, rect->width, rect->height);
       gtk_render_frame (context, cr, rect->x, rect->y, rect->width, rect->height);
       gtk_render_layout (context, cr, rect->x + OUTLINE_WIDTH, rect->y + OUTLINE_WIDTH,
                          priv->widget_name);
-      gtk_style_context_remove_class (context, "handle");
+      gtk_style_context_restore (context);
     }
 }
 
@@ -1084,6 +1084,7 @@ draw_selection (cairo_t *cr,
   if (alloc.x < 0 || alloc.y < 0) return;
 
   context = gtk_widget_get_style_context (parent);
+  gtk_style_context_save (context);
   gtk_style_context_add_class (context, "selection");
   r = color->red; g = color->green; b = color->blue;
   gtk_widget_translate_coordinates (widget, parent, 0, 0, &x, &y);
@@ -1122,7 +1123,7 @@ draw_selection (cairo_t *cr,
 
   /* Draw Selection box */
   gtk_render_frame (context, cr, x - left, y - top, w + left + right, h + top + bottom);
-  gtk_style_context_remove_class (context, "selection");
+  gtk_style_context_restore (context);
 }
 
 #define DIMENSION_OFFSET 9