narcissus-gpu: Assert when emitting barrier in render pass

narcissus-gpu: Assert when emitting barrier in render pass

Any use of `vkCmdPipelineBarrier` between `vkBeginRendering` and
`vkEndRendering` will be ignored, so assert for that case.

Avoid stuttering by using `cmd_encoder` to refer to the narcissus side
implementation of command buffers.