vo_gpu: desaturate after peak detection

[haasn]

This sacrifices some dynamic range for well-behaved sources, but
prevents catastrophic desaturation on badly mastered / too bright
sources. I think that's the better trade-off. This makes the
desaturation algorithm much "safer" to deploy by default, as well. One
could even argue going up to strength 1.0, which works better for some
sources but worse for others. But I think the current strength is the
best trade-off even after this change.

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I agree that my changes can be relicensed to LGPL 2.1 or later.

[jeeb]

I shall re-read the IRC discussion logs from yesterday night tomorrow, and then most likely merge this if it all seems sane enough.

[jeeb]

LGTM. The difference generally seems rather small in brightness. Things were a bit brighter before, now somewhat less bright. Will post some comparison shots.

[jeeb]

1. Kimi no Na Wa opening, --pause --start=47 (PQ, MaxCLL 777 nits, MaxFALL 370 nits)

   • Before
   • After

2. TravelXP sample seabed scene --pause --start=60 (HLG)

   • Before
   • After

Shin Godzilla sample clip I created earlier just fails to have mostly any effect, although that to be honest is easily the worst mastered piece of HDR content I own (even lacks metadata).