Pure CPU helper that lays out a group list into a contiguous indirect
buffer (DrawElementsIndirectCommand[]) and parallel BatchData[] —
opaque section first, transparent section second. Returns counts +
byte offset for the transparent section.
Tests cover: spec §5 walk-through layout; empty group list edge case;
ClipMap classification (treated as opaque, not transparent).
Static + public so tests can exercise without a GL context. Task 10
wires it into the rewritten Draw() method.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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