avutil/csp: add EOTF function definitions

The existing av_csp_trc_func_from_id() mostly implements the OETF, except for
PQ. As such, we are currently missing a precise definition of an ITU-R EOTF.
Introduce the new functions av_csp_itu_eotf() and av_csp_itu_eotf_inv(), to fill
this void. Note that this is not possible in all cases, e.g. AVCOL_TRC_LOG which
has no corresponding EOTF definition in any ITU-R standard.

Note that we cannot implement the proper HLG and SMPTE 428 OOTFs without access
to all three color channels, because they are not independent per channel. As a
result, we need to define them on double[3] instead of double (*func)(double).

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