UV mirror seams are now considered an attribute discontinuity. This is handled through a cheap trick of changing the tops bits of the material index per triangle.
This should not strictly be necessary because the QEM could account for the difference but to get it to properly catch issues in practice required increasing the UV error weight high enough that it impacts all meshes including ones that didn't have noticable issues. Targeting the mirror case specifically was better even though it shouldn't be special theoretically.
This fixes Nanite UV bugs in Frosty.
#jira UE-113150
#ROBOMERGE-SOURCE: CL 16411470 in //UE5/Private-Frosty/...
#ROBOMERGE-BOT: STARSHIP (Private-Frosty -> Main) (v804-16311228)
[CL 16412628 by brian karis in ue5-main branch]
12e43285f4