gif2webp: Use the default hint instead of WEBP_HINT_GRAPH.

This is much faster and the compression is slightly better too.

(cherry picked from commit c94ed49efd6265602f9423fdbc81c8106fda1069)

Change-Id: Ibf0d10eea83bfabfcc44ee497074767462ff41b1

examples/gif2webp.c

@@ -285,7 +285,6 @@ 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();