(Still adding tests: do not commit)
This fixes PR #632 so that:
(a) The 4 PerVertex builtins are added to an interface block for all stages except fragment.
(b) Other builtin qualified variables are added as "loose" linkage members.
(c) Arrayness from the PerVertex builtins is moved to the PerVertex block.
(d) Sometimes, two PerVertex blocks are created, one for in, one for out (e.g, for some GS that
both reads and writes a Position)
This commit adds support for copying nested hierarchical types of split
types. E.g, a struct of a struct containing both user and builtin interstage
IO variables.
When copying split types, if any subtree does NOT contain builtin interstage
IO, we can copy the whole subtree with one assignment, which saves a bunch
of AST verbosity for memberwise copies of that subtree.
