builtinicon: avoid calculating font-metrics in vast majority of cases

builtinicon: avoid calculating font-metrics in vast majority of cases

We perform lots of gadget allocations that require allocating a
GtkBuiltinIcon. One notable example is the scrollbar for a scrolled
window.

In the process of doing this, we often calculate baseline information that
isn't necessary. With how much this code path gets exercised, its worth
catching the result for the common case, which is that the font-description
has not changed and we are using the default language the application
was started with.

This simply caches the previous result and verifies that we can reuse it
with pango_font_description_hash() and a simple language check.

Numbers below are scrolling through a textview with GDK_KEY_Down.

Before:
      SELF CUMULATIVE    FUNCTION
[   0.08%] [   9.26%]    gtk_builtin_icon_get_preferred_size
[   0.01%] [   8.82%]      pango_context_get_metrics
[   0.02%] [   0.16%]      gtk_widget_get_pango_context
[   0.06%] [   0.06%]      pango_context_get_language
[   0.01%] [   0.02%]      g_type_check_instance_cast
[   0.02%] [   0.02%]      strlen
[   0.02%] [   0.02%]      pango_context_get_font_description
[   0.02%] [   0.02%]      g_list_foreach
[   0.01%] [   0.01%]      gtk_css_style_get_value
[   0.01%] [   0.01%]      itemize_with_font
[   0.01%] [   0.01%]      pango_context_get_type
[   0.01%] [   0.01%]      get_base_metrics
[   0.00%] [   0.01%]      pango_font_metrics_unref
[   0.01%] [   0.01%]      g_list_free
[   0.01%] [   0.01%]      gtk_builtin_icon_get_type

After:
      SELF CUMULATIVE    FUNCTION
[   0.08%] [   0.18%]    gtk_builtin_icon_get_preferred_size
[   0.02%] [   0.02%]      pango_font_description_hash
[   0.00%] [   0.02%]      gtk_widget_get_pango_context
[   0.00%] [   0.02%]        g_object_get_qdata
[   0.00%] [   0.02%]          g_datalist_id_get_data
[   0.02%] [   0.02%]      gtk_builtin_icon_get_type
[   0.01%] [   0.01%]      pango_context_get_font_description
[   0.00%] [   0.01%]      - - kernel - -
[   0.01%] [   0.01%]      pango_context_get_language
[   0.00%] [   0.01%]      gtk_css_style_get_value
[   0.00%] [   0.01%]      gtk_css_gadget_get_style

https://bugzilla.gnome.org/show_bug.cgi?id=765486