Speed up Guetzli on all-gray images.
Speed up Guetzli on all-gray images.

This has been causing timeouts for the fuzzer.

google/guetzli,guanzidashu/guetzli_ios_metal,dloeng/guetzli,ianhuang-777/guetzli,guanzidashu/guetzli_ios_metal,google/guetzli,ianhuang-777/guetzli,ianhuang-777/guetzli,szabadka/guetzli,dloeng/guetzli,szabadka/guetzli,szabadka/guetzli,dloeng/guetzli,guanzidashu/guetzli_ios_metal,guanzidashu/guetzli_ios_metal,google/guetzli,ianhuang-777/guetzli,google/guetzli