vo_opengl: implement ARIB STD-B68 (HLG) HDR TRC

This HDR function is unique in that it's still display-referred, it just
allows for values above the reference peak (super-highlights). The
official standard doesn't actually document this very well, but the
nominal peak turns out to be exactly 12.0 - so we normalize to this
value internally in mpv. (This lets us preserve the property that the
textures are encoded in the range [0,1], preventing clipping and making
the best use of an integer texture's range)

This was grouped together with SMPTE ST2084 when checking libavutil
compatibility since they were added in the same release window, in a
similar timeframe.

video/csputils.h

@@ -79,6 +79,7 @@ enum mp_csp_trc {
     MP_CSP_TRC_GAMMA28,
     MP_CSP_TRC_PRO_PHOTO,
     MP_CSP_TRC_SMPTE_ST2084,
+    MP_CSP_TRC_ARIB_STD_B67,
     MP_CSP_TRC_COUNT
 };