WRaster: add function to save image in memory

This patch adds the RSaveRawImage() function to the WRaster lib
to be able to save image structure in memory.
The WRaster lib version is bumped.

wrlib/load.c

@@ -162,6 +162,11 @@ RImage *RLoadImage(RContext *context, const char *file, int index)
 
 	assert(file != NULL);
 
+	/* just to suppress the compilation warning as index is only used with TIFF and GIF */
+#if !defined(USE_TIFF) && !defined(USE_GIF)
+	(void)index;
+#endif
+
 	if (RImageCacheSize < 0)
 		init_cache();