A merged mesh's side table entry (indexed scenes; one u32 per triangle,
by tree slot, four to a vec4u): bits 0-15 the source's slot in the
group's source table, bits 16-31 its material code (material + 1, 0: none).
The source table holds each source's instanceIndex, four to a vec4u.
Records keep both in the triangle instead (word 11, flags bits 16-31).
A merged mesh's side table entry (indexed scenes; one
u32per triangle, by tree slot, four to avec4u): bits 0-15 the source's slot in the group's source table, bits 16-31 its material code (material + 1, 0: none). The source table holds each source'sinstanceIndex, four to avec4u. Records keep both in the triangle instead (word 11, flags bits 16-31).