Defer requeuing buffers in v4l2 reader

Enqueuing buffers takes a long time, so we want to move it out of the
way of images being sent.

Requeue the buffers in a worker thread that's running at a lower
priority.

Signed-off-by: Ravago Jones <ravagojones@gmail.com>
Change-Id: I3887bca9099916b1864ff88893b940d6a31f7edf
diff --git a/frc971/vision/BUILD b/frc971/vision/BUILD
index c52afae..4dbd66c 100644
--- a/frc971/vision/BUILD
+++ b/frc971/vision/BUILD
@@ -76,6 +76,7 @@
         "//aos/events:epoll",
         "//aos/events:event_loop",
         "//aos/scoped:scoped_fd",
+        "//aos/util:threaded_consumer",
         "@com_github_google_glog//:glog",
         "@com_google_absl//absl/base",
     ],