Refresh screen at maximum rate based on timer

This handles window resizes much more smoothly as well.

src/app.d

@@ -15,6 +15,7 @@ GLuint program_id;
 mat4 view_matrix;
 int window_width;
 int window_height;
+private __gshared bool draw_pending;
 
 private void init()
 {
@@ -90,6 +91,7 @@ private void display(SDL_Window * window)
     glDrawElements(GL_TRIANGLE_FAN, 4, GL_UNSIGNED_SHORT, null);
 
     SDL_GL_SwapWindow(window);
+    draw_pending = false;
 }
 
 int main()
@@ -124,12 +126,13 @@ int main()
     }
 
     init();
+    SDL_AddTimer(5, &tick, null);
     resize(WIDTH, HEIGHT);
     SDL_Event event;
 outer:
     for (;;)
     {
-        if (SDL_PollEvent(&event))
+        if (SDL_WaitEvent(&event))
         {
             switch (event.type)
             {
@@ -159,8 +162,11 @@ outer:
                 break;
             }
         }
+        if (draw_pending)
+        {
             display(window);
         }
+    }
 
     SDL_GL_DeleteContext(context);
 
@@ -170,3 +176,16 @@ outer:
 
     return 0;
 }
+
+private extern(C) uint tick(uint interval, void * param) nothrow
+{
+    if (!draw_pending)
+    {
+        /* Draw a new frame every 5 ms if not still drawing the last. */
+        draw_pending = true;
+        SDL_Event event;
+        event.type = SDL_USEREVENT;
+        SDL_PushEvent(&event);
+    }
+    return interval;
+}