Found `unimplemented!` in a part of the code

[AlterionX]

Issue

I hit an unimplemented portion of command_buffers. It should probably be implemented.

[Rua]

This error seems to happen when you present a swapchain and then submit a command buffer. You can probably work around it by waiting for a fence in between.

Other than that, minor issues with futures are unlikely to be fixed, as we want to replace them with something else entirely.