(main)+threading for steps

Doing this experiment with threads shows a strong improvement in
render speed by separating "steps" from the direct render.

main.c

@@ -4,8 +4,12 @@
  * Description: Entry point of program
  */
 #include <malloc.h>
+#include <stdbool.h>
 #include <stdio.h>
 
+#include <pthread.h>
+#include <unistd.h>
+
 #include <raylib.h>
 #include <raymath.h>
 
@@ -17,6 +21,8 @@ struct State
 
   size_t window_len;
   int multiplier;
+
+  bool thread_alive;
 };
 
 typedef struct State state_t;
@@ -44,22 +50,32 @@ void step(state_t *state)
       }
 }
 
+void *compute_thread(void *input)
+{
+  state_t *state = input;
+  while (state->thread_alive)
+    step(state);
+  return NULL;
+}
+
 int main(void)
 {
-  state_t state    = {NULL, 256, 512, 0};
+  state_t state    = {NULL, 512, 512, 0, true};
   state.data       = calloc(state.dwidth * state.dwidth, sizeof(*state.data));
   state.multiplier = state.window_len / state.dwidth;
 
-  Camera2D camera = {0};
-  camera.zoom     = 1.0f;
+  const float zoom = 0.125f;
+  Camera2D camera  = {0};
+  camera.zoom      = 1.0f;
+
+  pthread_t step_thread;
+  pthread_create(&step_thread, NULL, &compute_thread, &state);
+
   InitWindow(state.window_len, state.window_len, "Abelian sand pile");
   SetTargetFPS(60);
-  const float zoom = 0.125f;
 
   while (!WindowShouldClose())
   {
-    step(&state);
-
     if (IsKeyPressed(KEY_UP) || IsKeyDown(KEY_UP))
     {
       Vector2 centre    = {state.window_len / 2, state.window_len / 2};
@@ -110,6 +126,9 @@ int main(void)
     EndMode2D();
     EndDrawing();
   }
+
+  if (state.thread_alive)
+    pthread_cancel(step_thread);
   CloseWindow();
   return 0;
 }