(Improvement) HDR infoframe replacement with proper BT2020 values

[Natrox]

Hello again, and a happy new year to all of you. Thank you for merging the earlier HDR feature.

I kept a close watch on discussion and experiences people had with HDR, and identified some key issues:

• Blacks may get crushed
• Very bright parts of an image will clip without scanlines
• It is impossible to get accurate colors.

After doing some research on the specification of the HDR infoframe, I constructed replacement metadata that matches with BT2020; the bytes for RGB primaries and whitepoint have been corrected for this color space. See the code for parameters used.

Results
People using hdr=1 will now get a more accurate color reproduction in BT2020 color space. The colors will more closely resemble those of SDR while still providing enhanced brightness and contrast. DCI P3 remains available by using hdr=2, but the metadata does not change and will still advertise itself as BT2020. hdr=3 is also usable but requires careful tweaking.

Recommendations
Since BT2020 is now accurate, I recommend users try out hdr=1 again - without any color control options (all default). Previously, a lot of users went with hdr=2 as it provided a better color reproduction.
When using any HDR option, please turn off the gamma filter in "Video Processing". Gamma is already set to 2.2 via the HDR infoframe.

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Thank you!