Vorbis dev - Mar 2021 - tremor lowmem branch, need advice for C99 variable-length arrays

Hello:

I attempted porting Tremor lowmem branch to use C99 variable-length
arrays, and it works for me. See the attached two patches:

- 0001-scope.patch: tweaks alloca() scope a little:
  Can anyone see any gotchas, any adverse effects I may have missed?

- 0002-vla.patch: ports to C99 VLAs as an option with alloca() still
  being the default:  Comments?
  I am not happy with the solution I used in mapping_inverse(), but
  it was the only solution I could find. Can anyone propose something
  better?

Thanks.

--
O.S.
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