Warn (once per call) about descriptors sharing an index — the later entry
silently wins in every index-keyed lookup and overwrites the earlier one's
texture slice, which corrupts rendering in hard-to-trace ways (e.g. a lava
slice bleeding into riverbed blending). Exported for reuse by
buildTerrainTextureArray, which accepts raw descriptors directly.
Warn (once per call) about descriptors sharing an index — the later entry silently wins in every index-keyed lookup and overwrites the earlier one's texture slice, which corrupts rendering in hard-to-trace ways (e.g. a lava slice bleeding into riverbed blending). Exported for reuse by buildTerrainTextureArray, which accepts raw descriptors directly.