icon theme: Improve loading of unthemed files

When creating icon info objects for unthemed files, we don't
really have a nominal size, so we pass 0 to mean 'load at
original size'. However, this is not what was happening.
To make this possible, add variants of some pixbuf loading
functions that take a scale factor instead of a desired size,
and use those when we don't have a nominal size.

gtk/gdkpixbufutilsprivate.h

@@ -0,0 +1,35 @@
+/* GTK - The GIMP Toolkit
+ * Copyright (C) 2016 Red Hat, Inc.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef __GDK_PIXBUF_UTILS_PRIVATE_H__
+#define __GDK_PIXBUF_UTILS_PRIVATE_H__
+
+#include <gdk-pixbuf/gdk-pixbuf.h>
+
+G_BEGIN_DECLS
+
+GdkPixbuf *_gdk_pixbuf_new_from_stream_scaled   (GInputStream  *stream,
+                                                 gdouble        scale,
+                                                 GCancellable  *cancellable,
+                                                 GError       **error);
+GdkPixbuf *_gdk_pixbuf_new_from_resource_scaled (const gchar   *resource_path,
+                                                 gdouble        scale,
+                                                 GError       **error);
+
+G_END_DECLS
+
+#endif  /* __GDK_PIXBUF_UTILS_PRIVATE_H__ */