twn_loop.c: place SDL_Delay() to not waste CPU time

src/twn_loop.c

@@ -252,6 +252,10 @@ static void main_loop(void) {
     /*       which for now is broken as glBufferData with NULL is used all over right after render */
     if (frames != 0)
         render();
+    else
+        /* don't waste clock cycles on useless work */
+        /* TODO: make it adjustable from config */
+        SDL_Delay((uint32_t)(ctx.desired_frametime - ctx.frame_accumulator) / 1250000);
 }