add a "magic" field to FloatImg descritor

floatimg.h

@@ -4,12 +4,13 @@
  *	http://la.buvette.org/photos/cumul
  */
 
-#define		FIMG_VERSION	132
+#define		FIMG_VERSION	133
 
 /*
  *	in memory descriptor
  */
 typedef struct {
+	int		magic;
 	int		width;
 	int		height;
 	int		type;