Swscale currently handles XYZ by embedding a forced conversion to
BT.709 RGB with a hardcoded matrix. This is not ideal, but to preserve the
status quo and avoid any unexpected changes in behavior, this patch merely
fixes the inferred primaries tag to match the reality.
In the future, I would like to handle XYZ properly, via direct conversion
to the target colorspace (or possibly simply by using a more fitting
RGB intermediate like SMPTE428), but for now just keep the status quo.
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