Use Cairo renderer for capturing image of widget

Do not rely on GPU renderers - Cairo renderer (SW renderer) is safe to use in all cases.

src/gtkchart.c

@@ -874,20 +874,17 @@ bool gtk_chart_save_png(GtkChart *chart, const char *filename)
  GdkPaintable *paintable = gtk_widget_paintable_new (GTK_WIDGET(chart));
  GtkSnapshot *snapshot = gtk_snapshot_new ();
  gdk_paintable_snapshot (paintable, snapshot, width, height);
- GskRenderNode *node = gtk_snapshot_to_node (snapshot);
- GdkSurface *surface = gdk_surface_new_toplevel (gdk_display_get_default());
- GskRenderer *renderer = gsk_renderer_new_for_surface (surface);
- gsk_renderer_realize (renderer, surface, NULL);
+ GskRenderNode *node = gtk_snapshot_free_to_node (snapshot);
+ GskRenderer *renderer = gsk_cairo_renderer_new ();
+ gsk_renderer_realize (renderer, NULL, NULL);
  GdkTexture *texture = gsk_renderer_render_texture (renderer, node, NULL);
  gdk_texture_save_to_png (texture, filename);
 
  // Cleanup
  g_object_unref(texture);
  gsk_renderer_unrealize(renderer);
  g_object_unref(renderer);
- g_object_unref(surface);
  gsk_render_node_unref(node);
- g_object_unref(snapshot);
  g_object_unref(paintable);
 
  return true;