Clang generate autological-unsigned-zero-compare warning.

[yunlia]

TOT clang generates warnings like
shader-source.cc:424:18: comparison of unsigned enum expression >= 0
is always true [-Werror,-Wtautological-unsigned-zero-compare]

The patch in the attachment fixes the issue

clang-autological-unsigned-zero-compare.txt

[afrantzis]

Thanks for the report. I have fixed this in a different way in aa17339, avoiding the problematic comparisons altogether.