use HINT_GRAPH as image_hint for gif source

quite rarely, it gives a different output
then without the HINT, but that's often for
a smaller size (tested with default -m and -m 6)

Change-Id: I51d221ab61f8e007983325031345728e8d80b241

examples/gif2webp.c

@@ -329,6 +329,7 @@ int main(int argc, const char *argv[]) {
     return -1;
   }
   config.lossless = 1;  // Use lossless compression by default.
+  config.image_hint = WEBP_HINT_GRAPH;   // always low-color
 
   if (argc == 1) {
     Help();