small change to https://github.com/nothings/stb/blob/master/deprecated/stretchy_buffer.txt so I can read it

vec.h

// vec
#define vec_free(a)         ((a) ? free(vec__raw(a)),0 : 0)
#define vec_push(a,v)       (vec__maybegrow(a,1), (a)[vec__n(a)++] = (v))
#define vec_count(a)        ((a) ? vec__n(a) : 0)
#define vec_add(a,n)        (vec__maybegrow(a,n), vec__n(a)+=(n), &(a)[vec__n(a)-(n)])
#define vec_last(a)         ((a)[vec__n(a)-1])

#include <stdlib.h>
#define vec__raw(a) ((int *) (a) - 2)
#define vec__m(a)   vec__raw(a)[0]
#define vec__n(a)   vec__raw(a)[1]

#define vec__needgrow(a,n)  ((a)==0 || vec__n(a)+n >= vec__m(a))
#define vec__maybegrow(a,n) (vec__needgrow(a,(n)) ? vec__grow(a,n) : 0)
#define vec__grow(a,n)  vec__growf((void **) &(a), (n), sizeof(*(a)))

static void vec__growf(void **arr, int increment, int itemsize)
{
   int m = *arr ? 2*vec__m(*arr)+increment : increment+1;
   void *p = realloc(*arr ? vec__raw(*arr) : 0, itemsize * m + sizeof(int)*2);
   assert(p);
   if (p) {
      if (!*arr) ((int *) p)[1] = 0;
      *arr = (void *) ((int *) p + 2);
      vec__m(*arr) = m;
   }
}