[SURF] Callbacks now use the INITIALIZER macros

[SURF] Callbacks now use the INITIALIZER macros

- Callbacks such as s_sg_platf_link_cbarg_t were not at all
  correctly initialized before; this was fixed!

- Several initializers used valid C99 initializer syntax; however,
  this is not supported by C++. Hence, these initializers were changed
  to simple lists. See also
  https://gcc.gnu.org/onlinedocs/gcc/Designated-Inits.html
  for more details on this issue.