Image: Fix adventurous use of function pointers
Nothing really wrong with it, as long as things go fine, except GCC 8
started warning about type mismatch of function pointers (as well as in
casts) more aggressively than before. To silence this warning, be
slightly less adventurous.
What we do with the lm_spec_burn_alpha function pointer is still a bit
odd though, maybe now even more than before.