colorspace: add PL_COLOR_TRC_LINEAR_{REL,ABS}

The internal representation of linear has black offset, but it is not expected for output, that expect black to be at 0.0, as linear is considered "black_scaled", same as other SDR transfers.

Note this is only for "SDR" linear, for more fancy things like scRGB, which is extended linear, we would need more adjustments to normalize both luminance and gamut to correct range. While for luminance it's trivial, for gamut we could need to remove clipping at mapping stage.