Incorrect normal map with material.side = THREE.DoubleSide + flatShading = true

[WestLangley]

Description

Normal maps are displayed incorrectly when material.side = THREE.DoubleSide + flatShading = true

Broken with #25693 (confirmed with git bisect).

Related bug: #25701

Reproduction steps

1. Create a double-sided material having a normal map and flat shading set to true
2. Extrusions on one side should appear as indentations on the other side

r156