color: add TF enumeration, advertise set_tf_power

Here is the first example of adding an enum in the XML for H.273 defined code points.

@emersion, @daniels, @joshuaashton, is this what you had in mind? And who else wanted this?

It results in names like WP_IMAGE_DESCRIPTION_FACTORY_PARAMETRIC_V1_TF_CICP_ST2084 (perceptual quantizer).

Personally I find this enum with its documentation more confusing than helping, but maybe I didn't choose the doc comments you were thinking of.

I have no plans to make use of this enum in Weston, the const struct pixel_format_info * approach looks more attractive.