Stormwind's walls, trim and roofs are DXT5 with an unused alpha channel — mean
alpha 17/255, every texel below the cutoff. The renderer infers an alpha cutout
for an opaque batch whenever its texture "has alpha", a rescue for character
textures, so all of Stormwind was being discarded and the sky showed through the
buildings with speckle where a stray texel cleared the threshold.
Mark backdrops as scene models and take them at their word: only an alpha-key
material (blendMode 1) cuts out. The hair-geoset and skin-composite guesses are
skipped for them too — a scene's submesh ids are all 0, which the hair guess reads
as a hair geoset for every batch.
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