Make night sky rotation accurate and deterministic

Fixes #9357 (closed)

Night sky rotation appears to have been based on a combination of timescale and time elapsed (since application start) for the last 11 years. The main consequence is that a save doesn't keep its rotation, which can be obvious in screenshots and imported save comparisons.

This MR calculates the rotation through the save's gametime and also fixes the rotation direction. I've verified it with imported saves:

Morrowind.exe:

OpenMW with fix:

OpenMW (master):

Screenshots include the moon size fix from !5608 (merged), but not the rest of the changes.

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