Evas ETC2: Remove some C99/GCC extensions code

Evas ETC2: Remove some C99/GCC extensions code

There is still some C99 code in the file in the form of
  for (int k = 0; ...)

If there's a strong requirement not to use this form, I'll
change it, otherwise I find this specific code style more
readable (k is local to this iteration).

This patch and the previous one even give a ~10% speedup
on the encoding time. Sweet :)