The benefit here is that we can now properly cross-fade when one of
start/end is fully clipped out by just replacing it with an opacity op
for the other.
This was not possible with the old way we did things.
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().
GskVulkanOp is meant to be a proper abstraction of operations
the Vulkan renderer will be doing.
For now it's an atrocious clunky piece of junk wedged into the
renderpass codebase.
It's so temporary that I didn't even adjust indentation of the code.
