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Fix T93467: Use world bg color for pose library previews

Use the World viewport color when rendering pose library previews.

The World's viewport color is chosen instead of the World shading nodes,
as the latter would require rendering with `OB_RENDER` (instead of
`OB_SOLID`), which would take considerably longer.

Manifest Task: T93467

Reviewed By: sybren

Differential Revision: https://developer.blender.org/D13470