Compiling errors for apng fixed

If I'm looking at the GitHub right these commits still failed to build on the buildbots.

/home/travis/build/STJr/Kart-Public/src/apng.c:283:23: error: incompatible
      function pointer types assigning to 'apng_set_acTL_ptr' (aka 'unsigned
      long (*)(struct png_struct_def *, struct png_info_struct *, unsigned long,
      unsigned long)') from 'void (*)(png_structp, png_infop, apng_infop,
      png_uint_32, png_uint_32)' (aka 'void (*)(struct png_struct_def *, struct
      png_info_struct *, struct apng_info_def *, unsigned long, unsigned long)')
      [-Werror,-Wincompatible-function-pointer-types]
                ainfop->set_acTL_fn = &apng_set_acTL_dummy;
                                    ^ ~~~~~~~~~~~~~~~~~~~~

Depending what version of libpng, the function will have diff set of params

added 1 commit

• 46edc1e3 - Create the correct dummy function

Compare with previous version

Not version problem--me confusing libpng apng functions with my own. eek

I may as well note here: apng_set_write_fn have an extra whitespace

added 1 commit

• c74b05ea - Remove extra whitespace

Compare with previous version

Doesn't generate warnings anymore on my end.

Let merge this and see what happens on a system that does not have aPNG support in libpng

merged

mentioned in commit a6315f89

mentioned in commit james/SRB2@7cd60a54