renderpass was writing timestamp before renderpass activity was complete.
MVKCommandBuffer tracks whether it contains a stage-based timestamp command,
and MVKCommandEncoder updates the timestamp command fence when ending any
Metal command encoder on such a MVKCommandBuffer.
MVKCommandEncoder reorder member variables to avoid layout gaps (unrelated).
MVKCommandBuffer update _commandCount even for single-use immediate command
encoding (unrelated).
e2168a33db