Expose Previous Frame Sync

[freesig]

Problem

Currently there is no way to get hold of the fence that is signaled by the gpu when the previous frame (and it's command buffer) is submitted.

Motivation

We need to be able to know when the previous command buffer is done so that we can tell that any copys that we added to the previous frame are done. Then we know it's safe to access the buffers associated with these copys.

Solution

We basically need a good way to expose pub(crate) previous_frame_end: Mutex<Option<vk::FenceSignalFuture<Boxvk::GpuFuture>>>,