Improve handling of motion vectors for multimesh instances.

Fixes #67287. There was a subtle error where due to how enabling motion vectors for multi-meshes was handled, only the first instance would have a valid transforms buffer and the rest would point to an invalid buffer. This change moves over the responsibility of enabling motion vectors only when changes happen to the individual 3D transforms or the entire buffer itself. It also fixes an unnecessary download of the existing buffer that'd get overwritten by the current cache if it exists. Another fix is handling the case where the buffer was not set, and enabling motion vectors would not cause the buffer to be recreated correctly.

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