The libvmaf filter was doing substring checks in place of string equality
comparisons. This led to a bug when the user specified the pooling method
"harmonic_mean", since "mean" was checked first and the substring comparison
returned true. This patch changes all substring comparisons for string equality
comparisons. This is both correct and more efficient than the existing method.
Signed-off-by: nilfm <nilf@netflix.com>