RFC: d3d12: reset submitted batches after successfully waiting for a d3d12_fence

A batch is usually only reset when it is newly started. However, when we resize the swapchain, all resources used therein must not be referenced any more, so we need to find a suitable moment to reset the batches after flushing. Since before resizing the swapchain we actually do a flush and wait for the returned fence, screen->fence_finish seems to be a good place to exactly, reset all submitted batches when we have successfully waited for the fence.

This approach assumes that no new batch can be sumbitted while we wait for the fence.