vulkan: Emit a renderpass op

vulkan: Emit a renderpass op

... instead of doing the equivalent things manually by creating a
RenderPass and calling the relevant functions.

Now all renderpass operations are indeed stored in ops.

Also reshuffle the command emission code, because we no longer need to
emit the ops for the base renderpass.

As a result we only submit a single command buffer containing all the
render passes instead of once per render pass.
We also bind vertex buffers and descriptor sets only once now at the
start instead of once per renderpass.