prepareRender() re-uploaded every animated M2 instance's bone matrices
into the mega bone SSBO every frame — including the majority whose
bones were never recomputed because they are distance, frustum, or
frame-skip culled and keep their previous matrices. The bonesDirty
flag computeBoneMatrices() sets was never consumed. Track the slot
each instance last uploaded to per frame index and memcpy only when
the bones changed or the slot moved, cutting megabytes of redundant
host-visible writes per frame in doodad-dense scenes.
The selection circle also re-ran terrain/WMO/M2 floor raycasts every
frame; reuse the result while the target's position is unchanged,
refreshing every 30 frames so streamed-in geometry still snaps.
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