Compute chapter still uses fences

Just noticed that while the reworked tutorial made the compute chapter use timeline semaphores it's still using fences to wait on the host side. But timeline semaphores can replace that, and so we should do that in the tutorial.

This is what I do in my code to replace fences with timeline semaphores:

```cpp
VkSemaphoreWaitInfo semaphoreWaitInfo{ .sType = VK_STRUCTURE_TYPE_SEMAPHORE_WAIT_INFO };
semaphoreWaitInfo.semaphoreCount = 1;
semaphoreWaitInfo.pSemaphores = &timeLineSemaphore.handle;
semaphoreWaitInfo.pValues = &timeLineSemaphore.value;
vkWaitSemaphoresKHR(device, &semaphoreWaitInfo, UINT64_MAX);
```