HiDPI: use original notify callback signature

zathura/callbacks.c

@@ -217,14 +217,15 @@ cb_refresh_view(GtkWidget* GIRARA_UNUSED(view), gpointer data)
 }
 
 void
-cb_scale_factor(GtkWidget* widget, GParamSpec* UNUSED(pspec), zathura_t* zathura)
+cb_scale_factor(GObject* object, GParamSpec* UNUSED(pspec), gpointer data)
 {
-  if (widget == NULL || zathura == NULL || zathura->document == NULL) {
+  zathura_t* zathura = data;
+  if (zathura == NULL || zathura->document == NULL) {
     return;
   }
 
 #ifdef HAVE_HIDPI_SUPPORT
-  int new_factor = gtk_widget_get_scale_factor(widget);
+  int new_factor = gtk_widget_get_scale_factor(GTK_WIDGET(object));
 #else
   int new_factor = 1;
 #endif

zathura/callbacks.h

@@ -85,11 +85,11 @@ void cb_refresh_view(GtkWidget* view, gpointer data);
  *
  * It records the new value and triggers a re-rendering of the document.
  *
- * @param widget The view widget
+ * @param object The view widget
  * @param pspec The GParamSpec for the scale-factor property
- * @param zathura The zathura instance
+ * @param gpointer The zathura instance
  */
-void cb_scale_factor(GtkWidget* widget, GParamSpec *pspec, zathura_t* zathura);
+void cb_scale_factor(GObject* object, GParamSpec* pspec, gpointer data);
 
 /**
  * This function gets called when the value of the "pages-per-row"