wrlib: Added clean-up of library internals in 'RShutdown'

The library uses internally a cache of tables to convert image to different
depths, there is now an internal function 'r_destroy_conversion_tables' to
free them.

wrlib/convert.h

@@ -0,0 +1,39 @@
+/*
+ * Raster graphics library
+ *
+ * Copyright (c) 2014 Window Maker Team
+ *
+ *  This library is free software; you can redistribute it and/or
+ *  modify it under the terms of the GNU Library 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
+ *  Library General Public License for more details.
+ *
+ *  You should have received a copy of the GNU Library General Public
+ *  License along with this library; if not, write to the Free
+ *  Software Foundation, Inc., 51 Franklin St, Fifth Floor, Boston,
+ *  MA 02110-1301, USA.
+ */
+
+/*
+ * Functions to convert images to given color depths
+ *
+ * The functions here are for WRaster library's internal use only,
+ * Please use functions in 'wraster.h' in applications
+ */
+
+#ifndef WRASTER_CONVERT_H
+#define WRASTER_CONVERT_H
+
+
+/*
+ * Function for to release internal Conversion Tables
+ */
+void r_destroy_conversion_tables(void);
+
+
+#endif