Fix bug with PPM writing

src/main.c

@@ -60,7 +60,7 @@ int main(int argc, char *argv[]) {
   char max[] = {'2', '5', '5', '\n'};
   fwrite(max, sizeof(max), 1, out);
 
-  for (u64 i = 0; i < img->buf_length; i += 4) {
+  for (u64 i = 0; i < img->pixel_count; ++i) {
     fwrite(&(img->data[i]), sizeof(u8), 3, out);
   }