stb_image_write.h: Allow setting custom zlib compress function for PNG

The builtin stbi_zlib_compress does not compress as well as zlib or
miniz (which is not too surprising as it's <200 LOC), thus PNGs created
by stb_image_write are about 20-50% bigger than PNGs compressed with
libpng.
This change lets the user supply a custom deflate/zlib-style compress
function, which improves compression a lot. This was requested in #113.

Example for zlib:

#include <zlib.h>
unsigned char* compress_for_stbiw(unsigned char *data, int data_len,
                                  int *out_len, int quality)
{
  uLongf bufSize = compressBound(data_len);
  // note that buf will be free'd by stb_image_write.h
  // with STBIW_FREE() (plain free() by default)
  unsigned char* buf = malloc(bufSize);
  if(buf == NULL)  return NULL;
  if(compress2(buf, &bufSize, data, data_len, quality) != Z_OK)
  {
    free(buf);
    return NULL;
  }
  *out_len = bufSize;

  return buf;
}
#define STBIW_ZLIB_COMPRESS compress_for_stbiw
#define STB_IMAGE_WRITE_IMPLEMENTATION
#include "stb_image_write.h"
// ...

stb_image_write.h

@@ -16,16 +16,22 @@ ABOUT:
    adapted to write to memory or a general streaming interface; let me know.
 
    The PNG output is not optimal; it is 20-50% larger than the file
-   written by a decent optimizing implementation. This library is designed
-   for source code compactness and simplicity, not optimal image file size
-   or run-time performance.
+   written by a decent optimizing implementation; though providing a custom
+   zlib compress function (see STBIW_ZLIB_COMPRESS) can mitigate that.
+   This library is designed for source code compactness and simplicity,
+   not optimal image file size or run-time performance.
 
 BUILDING:
 
    You can #define STBIW_ASSERT(x) before the #include to avoid using assert.h.
    You can #define STBIW_MALLOC(), STBIW_REALLOC(), and STBIW_FREE() to replace
    malloc,realloc,free.
-   You can define STBIW_MEMMOVE() to replace memmove()
+   You can #define STBIW_MEMMOVE() to replace memmove()
+   You can #define STBIW_ZLIB_COMPRESS to use a custom zlib-style compress function
+   for PNG compression (instead of the builtin one), it must have the following signature:
+   unsigned char * my_compress(unsigned char *data, int data_len, int *out_len, int quality);
+   The returned data will be freed with STBIW_FREE() (free() by default),
+   so it must be heap allocated with STBIW_MALLOC() (malloc() by default),
 
 USAGE:
 
@@ -650,6 +656,7 @@ int stbi_write_hdr(char const *filename, int x, int y, int comp, const float *da
 // PNG writer
 //
 
+#ifndef STBIW_ZLIB_COMPRESS
 // stretchy buffer; stbiw__sbpush() == vector<>::push_back() -- stbiw__sbcount() == vector<>::size()
 #define stbiw__sbraw(a) ((int *) (a) - 2)
 #define stbiw__sbm(a)   stbiw__sbraw(a)[0]
@@ -730,8 +737,14 @@ static unsigned int stbiw__zhash(unsigned char *data)
 
 #define stbiw__ZHASH   16384
 
+#endif // STBIW_ZLIB_COMPRESS
+
 unsigned char * stbi_zlib_compress(unsigned char *data, int data_len, int *out_len, int quality)
 {
+#ifdef STBIW_ZLIB_COMPRESS
+   // user provided a zlib compress implementation, use that
+   return STBIW_ZLIB_COMPRESS(data, data_len, out_len, quality);
+#else // use builtin
    static unsigned short lengthc[] = { 3,4,5,6,7,8,9,10,11,13,15,17,19,23,27,31,35,43,51,59,67,83,99,115,131,163,195,227,258, 259 };
    static unsigned char  lengtheb[]= { 0,0,0,0,0,0,0, 0, 1, 1, 1, 1, 2, 2, 2, 2, 3, 3, 3, 3, 4, 4, 4,  4,  5,  5,  5,  5,  0 };
    static unsigned short distc[]   = { 1,2,3,4,5,7,9,13,17,25,33,49,65,97,129,193,257,385,513,769,1025,1537,2049,3073,4097,6145,8193,12289,16385,24577, 32768 };
@@ -833,6 +846,7 @@ unsigned char * stbi_zlib_compress(unsigned char *data, int data_len, int *out_l
    // make returned pointer freeable
    STBIW_MEMMOVE(stbiw__sbraw(out), out, *out_len);
    return (unsigned char *) stbiw__sbraw(out);
+#endif // STBIW_ZLIB_COMPRESS
 }
 
 static unsigned int stbiw__crc32(unsigned char *buffer, int len)