Reduce priority of enqueue thread

It was higher priority than the april tag detection code, but has much
longer deadlines.  Lower the priority.

Change-Id: I8746c31cc747624d46c6a5b03bad702fe084c81f
Signed-off-by: Austin Schuh <austin.linux@gmail.com>
diff --git a/frc971/vision/v4l2_reader.cc b/frc971/vision/v4l2_reader.cc
index 7c0546f..2553003 100644
--- a/frc971/vision/v4l2_reader.cc
+++ b/frc971/vision/v4l2_reader.cc
@@ -327,7 +327,8 @@
     : V4L2ReaderBase(event_loop, device_name),
       epoll_(epoll),
       image_sensor_fd_(open(image_sensor_subdev.c_str(), O_RDWR | O_NONBLOCK)),
-      buffer_requeuer_([this](int buffer) { EnqueueBuffer(buffer); }, 20) {
+      buffer_requeuer_([this](int buffer) { EnqueueBuffer(buffer); },
+                       kEnqueueFifoPriority) {
   PCHECK(image_sensor_fd_.get() != -1)
       << " Failed to open device " << device_name;
   StreamOn();