Fix buffer readback on vulkan not advancing source offset for copies
Fix buffer readback on vulkan not advancing source offset for copies

* This would affect anything that displayed more than 16MB of buffer data in the
  UI, as well as replay proxying with the remote server displaying more than
  16MB of buffer data.
TurtleRockStudios/renderdoc_public,googlestadia/renderdoc,googlestadia/renderdoc,baldurk/renderdoc,Zorro666/renderdoc,googlestadia/renderdoc,baldurk/renderdoc,Zorro666/renderdoc,Zorro666/renderdoc,baldurk/renderdoc,googlestadia/renderdoc,TurtleRockStudios/renderdoc_public,googlestadia/renderdoc,baldurk/renderdoc,googlestadia/renderdoc,TurtleRockStudios/renderdoc_public,baldurk/renderdoc,TurtleRockStudios/renderdoc_public,Zorro666/renderdoc,TurtleRockStudios/renderdoc_public,baldurk/renderdoc,Zorro666/renderdoc,Zorro666/renderdoc,TurtleRockStudios/renderdoc_public