fix(VideoWriter): close depth_writer in close() method

[billybjork]

Summary

The VideoWriter class was not closing the depth video writer in its close() method, causing depth map video files (.depth.mp4) to be corrupted/unplayable.

Problem

When using VideoWriter with render_depth=True, the depth video files are never finalized because close() only closes image_writer:

def close(self):
    """Finish writing."""
    self.image_writer.close()
    # depth_writer is never closed!

The imageio writer requires close() to be called to write the video file headers and finalize the container.

Fix

1. Initialize self.depth_writer = None before the conditional assignment (also fixes a potential AttributeError if render_depth=False and code elsewhere checks self.depth_writer)
2. Close depth_writer in the close() method if it exists

Testing

Verified that depth map videos are now playable after this fix.