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/Makefile.am

@@ -28,6 +28,7 @@ libwraster_la_SOURCES = 	\
 	save.c		\
 	gradient.c 	\
 	xpixmap.c	\
+	convert.h 	\
 	convert.c 	\
 	context.c 	\
 	misc.c 		\