Newer specifications such as H.273 have, and probably will further
in the future add new values to these, so - instead of trying to update
these limits - we should simply check if the values are not set to the
value of "unspecified".

This should allow newer avutil values such as IPT-C2 or YCgCo-R
variants be passed to x265 itself, which apparently does its own
validation further down the line.