buffer: work with the block structure instead of the node
buffer: work with the block structure instead of the node

This has several benefits:
- the block structure is more scoped than the node structure
- it also makes the code more consistent with the approach taken in
  geometry and pass to access the fields information
- it allows in the future to honor the field offset
- we remove the need for accessing the block_opts structure (which will
  be scoped away in a future commit)

gopro/gopro-lib-node.gl,gopro/gopro-lib-node.gl,gopro/gopro-lib-node.gl,gopro/gopro-lib-node.gl