EuRoC Recorder

Adds a way to record euroc datasets from frame/IMU sources.

It exposes an xrt_slam_sinks* for which each sink is in charge of saving the samples to disk. The recorder has two queued stages that work in separate threads; first it copies the frames to memory (faster than writing to disk) so that it can release the original frame as soon as possible, and then it writes that copy to disk.

To implement this, u_sink_queue needed to be extended to support more than one element.