libvmaf API does not support `perc*` pool methods

The libvmaf API used by ffmpeg does not support the perc5, perc10, perc20 pooling methods. Furthermore attempting to pass one of these values will silently fallback to mean as the pooling method due to the implementation in pool_method_map, leading to a confusing user experience (even though the documentation mentions that only min, mean and harmonic_mean are supported).

It would be great to either unify the pooling methods such that perc* works, or to perhaps consider returning an error if an unsupported pooling method is requested.