Shamelessly stole a write_to_png routine from abelian-sandpile

Found and fixed a bug I would've never noticed in abelian sandpile:
the png is technically rotated by 90 using the standard WIDTH then
HEIGHT for loop, so I had to switch them.  As the abelian sandpile
fractal is 4 way symmetric this wasn't noticeable.

main.c

@@ -16,6 +16,8 @@
 #include <raylib.h>
 #include <raymath.h>
 
+#include <stb/stb_image_write.h>
+
 #define WIDTH       1024
 #define HEIGHT      1024
 #define MAX_THREADS 4
@@ -90,6 +92,25 @@ void threads_cancel_render(void)
     pthread_join(threads[i], NULL);
 }
 
+bool write_to_png(const char *filepath)
+{
+  unsigned char *image_data = calloc(3 * WIDTH * HEIGHT, sizeof(*image_data));
+
+  size_t image_ptr = 0;
+
+  for (size_t i = 0; i < HEIGHT; ++i)
+    for (size_t j = 0; j < WIDTH; ++j, image_ptr += 3)
+    {
+      Color c                 = cells[(j * WIDTH) + i];
+      unsigned char colour[3] = {c.r, c.g, c.b};
+      memcpy(image_data + image_ptr, colour, sizeof(*colour) * 3);
+    }
+
+  stbi_write_png(filepath, WIDTH, HEIGHT, 3, image_data, 3 * WIDTH);
+  free(image_data);
+  return true;
+}
+
 #define ZOOM_INC 0.3f
 #define MOVE_INC WIDTH / 50
 int main(void)