dep/stb: Use zlib for png compression

dep/stb/src/stb_image_write.c

@@ -1,2 +1,28 @@
+#include <stdlib.h>
+
+#include "zlib.h"
+
+// https://github.com/nothings/stb/issues/113
+static unsigned char* compress_for_stbiw(unsigned char* data, int data_len, int* out_len, int quality)
+{
+  const 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"