Expose sunlight direction to post-processing (#8826)

Alexei Kotovrequested to merge
Capostrophic/openmw:sunlightpos into master

Closes #8826 (closed).
Introduces omw.sunVec to the post-processing API. That's how sunlight direction is exposed in MGE XE. MGE XE only inverts the sun disc position, the direction is exposed as-is.

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