Writes an unmerged indexed mesh's header (its vertex base, and with
codes the material table's distance) and its material table (codes:
each original triangle's material code, by original index) into words
(4 per 16-byte unit), the mesh's first triangle at unit triangleOffset.
The region must be indexedLeadingUnits(n, 0, codes !== null) units.
Writes an unmerged indexed mesh's header (its vertex base, and with
codesthe material table's distance) and its material table (codes: each original triangle's material code, by original index) intowords(4 per 16-byte unit), the mesh's first triangle at unittriangleOffset. The region must be indexedLeadingUnits(n, 0, codes !== null)units.