Show '*' in file name when it has changed

Will need to make sure this does not affect the performances, since this
work by computing the hash of the image at each frame!

src/goxel.c

@@ -427,7 +427,13 @@ void goxel_release_graphics(void)
 static void update_window_title(void)
 {
     char buf[1024];
-    sprintf(buf, "Goxel %s%s %s", GOXEL_VERSION_STR, DEBUG ? " (debug)" : "",
+    bool changed;
+
+    changed = image_get_key(goxel.image) != goxel.image->saved_key;
+    sprintf(buf, "Goxel %s%s %s%s",
+            GOXEL_VERSION_STR,
+            DEBUG ? " (debug)" : "",
+            changed ? "*" : "",
             goxel.image->path ?: "");
     sys_set_window_title(buf);
 }