The implementation doesn't share much with get.c because:
  * the refactoring needed for get.c to not depend on ctx->Array.VAO would
    be quite large
  * glGetVertexArray* would still need to filter pname to only accept the one
    specified by the spec
  * these functions are getter, the implementation is trivial (the complexity
    is in the correct filtering of pname input)

Reviewed-by: Marek Olšák <marek.olsak@amd.com>