Remove --rec2020 option; set encoder param automatically from the DFD values

[MarkCallow]

if the input .exr file has bt.2020 chromaticities. The loader already matches the chromaticities attribute, if present, to a KHR_DF_PRIMARIES value.

[richgel999]

Yes, this is crucial. The ITP/Delta E ITP colorspace calculations are different for 2020. (Different enough to make a difference.)

[MarkCallow]

We must remove the --rec2020 option from encode and simply set the corresponding API option in the Binomial encoder params if the ktxTexture2 being encoded has primaries of KHR_DF_PRIMARIES_BT2020. If a user needs to assign this when creating the .ktx2 they can use the --assign-primaries or --convert-primaries options to ktx create. For the most part these should be set automatically from file metadata.

Having a separate --rec2020 option will lead to confusion and mistakes.

I have edited the issue title to indicate this.

[MarkCallow]

We should probably keep the bool rec2020 in ktxBasisParams so programmers have full control but when calling the Binomial encoder, the equivalent value must be set if the ktxTexture2 being encoded has BT.2020 primaries. The value in ktxBasisParams should be used as an override only and the documentation in ktx.h must be changed to indicate that. Need to confirm that the imageio input plugins all set the primaries if the file metadata indicates BT.2020.