Tonemapping for HDR screenshare

[njdom24]

After #11623, HDR screensharing now works, but it's too washed-out to really be usable.

IMO the existing behavior when moving an HDR game to an SDR monitor looks more acceptable, so it'd be cool if that transformation could be used in screen capture.

For now, can be worked around by taking a ~25% performance hit using wl-mirror on an 8-bit headless display, which falls back to a CPU copy mode, but does look okay.

[vaxerski]

HDR screensharing was never looking right, because it seems that our HDR -> SDR pipeline is wonk.

If you disable HDR and use wide or edid cm, it should work fine.