Flattening doesn't play well with dynamic indices. In this case, it's
better to leave it as an array of structs.
(I wanted to do this for named blocks generally. Trouble is, the builtin
`gl_out` block is *also* a named block...)
Fixes six more CTS tests, under
`dEQP-VK.tessellation.user_defined_io.per_patch_block_array.*`.
