Per-frame callback that returns the active skinned draw items. The
engine doesn't yet bucket SkinnedMesh objects from the scene-graph
traversal, so the app supplies them directly.
Optional per-frame GPU-instanced batches (repeated skinned geometry sharing
a mesh+material+palette — e.g. a level's reused mesh library). Drawn after
drawItems into the same GBuffer.
Per-frame callback that returns the active skinned draw items. The engine doesn't yet bucket SkinnedMesh objects from the scene-graph traversal, so the app supplies them directly.