wrlib: Added clean-up of image cache in 'RShutdown'

The library maintains a cache of the images loaded, which is now
emptyed when the lib is asked to shutdown.

wrlib/misc.c

@@ -251,4 +251,5 @@ void RShutdown(void)
 #ifdef USE_MAGICK
 	RReleaseMagick();
 #endif
+	RReleaseCache();
 }