Instead of creating a pipeline GObject, just ask for the VkPipeline.
And instead of having the Op handle it, just let the renderpass look
up/create the relevant pipeline while creating commands so that it can
insert vkCmdBindPipeline calls as-needed.
Instead of creating the op manually, just pass in the renderpass and
have the op created from there.
This way ops aren't really initialized anymore, they are more appended
to the queue, so instead of foo_op_init() we can just call the function
foo_op().
