vulkan: Add VulkanOp->next

vulkan: Add VulkanOp->next

Set it after creating all the ops and then use it for iterating.

Note that we cannot set it while creating the ops because the array may
be realloc()ed into a different memory region which would invalidate all
the pointers.

It currently has no use, but that will come later.

Also put the typedefs into headers in gsk/vulkan, they have nthing to do
outside that directory.