We now store all the relevant state of the image inside the VulkanImage
struct, so we can delay barriers for as long as possible.
Whenever we want to use an image, we call the new
gsk_vulkan_image_transition() and it will add a barrier to the desired
state if one is necessary.
This op queues a download of an image. The image will only be available
once the commands finished executing, so it requires waiting for the
render to finish, which makes the API a bit awkward.
Included is also a download_png_op() useful for debugging.
