[gnome-calculator] Fix use of GtkStyleContext for GTK+ 3.20

[Michael Catanzaro <mcatanzaro src gnome org>]

commit f1964266f1e0206baa5c9101fbfb30a62931fef5
Author: Michael Catanzaro <mcatanzaro igalia com>
Date:   Sun Feb 14 11:54:04 2016 -0600

    Fix use of GtkStyleContext for GTK+ 3.20

 src/math-display.vala |    8 +++++++-
 1 files changed, 7 insertions(+), 1 deletions(-)
---
diff --git a/src/math-display.vala b/src/math-display.vala
index a45533b..c52f6e3 100644
--- a/src/math-display.vala
+++ b/src/math-display.vala
@@ -46,7 +46,13 @@ public class MathDisplay : Gtk.Viewport
         /* TEMP: Disabled for now as GTK+ doesn't properly render a right aligned right margin, see bug 
#482688 */
         /*source_view.set_right_margin (6);*/
         source_view.set_justification (Gtk.Justification.RIGHT);
-        var font_desc = source_view.get_style_context ().get_font (Gtk.StateFlags.NORMAL);
+
+        var style_context = source_view.get_style_context ();
+        style_context.save ();
+        style_context.set_state (Gtk.StateFlags.NORMAL);
+        var font_desc = style_context.get_font (Gtk.StateFlags.NORMAL);
+        style_context.restore ();
+
         font_desc.set_size (16 * Pango.SCALE);
         source_view.override_font (font_desc);
         source_view.set_name ("displayitem");