GtkTextView: Use *some* extra size for horizontal scrolling

Horizontal scrolling is unusual, but specifying some extra offscreen
space for it in free in the normal case where the viewport is the
same width as the canvas anyway, so lets do it.

gtk/gtktextview.c

@@ -3701,7 +3701,7 @@ gtk_text_view_size_allocate (GtkWidget *widget,
 
   DV(g_print(G_STRLOC"\n"));
 
-  _gtk_pixel_cache_set_extra_size (priv->pixel_cache, 0,
+  _gtk_pixel_cache_set_extra_size (priv->pixel_cache, 64,
                                    allocation->height / 2);
 
   gtk_widget_get_allocation (widget, &widget_allocation);