descale Add kernel used as frameprop

[LightArrowsEXE]

It would be nice if the user could very easily see what kernel was used in the end from within the output clip. This could be easily done through a frameprop. The alternative, as far as I'm aware, is to grab the DescaleAttempts from DescaleResult, but that's a bit annoying if you're trying to quickly see what kernels were used for a whole bunch of different scenes.