Hash VMAF args for cache

[t-nil]

Changing VMAF args such as model, pool - even resolution - results in different scores. These should therefor be included in the cache key.

Things still not reflected:

• Change in VMAF version
• Change in default model used by VMAF (because ab-av1 doesn't set it and I see no trivial way to detect it)
• Change in model used hardcoded by ab-av1 (if that changes, we still could invalidate all the hashes since it's very likely that none of the scores match anymore)

[alexheretic]

Good spot thanks!