EDITED
I have now tested & confirmed, SoftTH spreads this game better than native support. Rendered cutscenes are full hor+. There is no drawback to using SoftTH, it just makes game better.
Attached below are the two SoftTH files you need, zipped with instruction on where they go. Files will support 21:9 (2560x1080 or 3440x1440).
Proof (blog post also updated):
(rendered cutscene, full hor+ with SoftTH)
However, some people may have a problem in Win10. If so, respond to discuss fix.
Attachment:
SoftTH 21_9 files (corrected).zip [422.91 KiB]
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(sorry there was a mistake in previous attachment's 3440x1440, now fixed)