Screenshot hang with Vulkan

[hrydgard]

After #19126 , the Vulkan backend hangs when you take a screenshot, if multithreading is enabled.

The problem seems to be something like after the screenshot, finishing the frame without any present command buffer, leading to the fence not getting unlocked.

I think the workaround in VulkanQueueRunner RunSteps on line 380 is likely in the wrong location.

Thanks to VeryMelon for reporting.