CameraServer: Add switched camera support (#1600)

This adds a new function "addSwitchedCamera" that creates and publishes a
virtual camera where the published stream information is consistent even if
the mjpeg server source is switched to a different camera.

Previously, changing the source of the mjpeg server resulted in updating the
stream information published for that source.

cameraserver/src/main/native/include/cameraserver/CameraServer.h

@@ -173,6 +173,14 @@ class CameraServer {
   cs::AxisCamera AddAxisCamera(const wpi::Twine& name,
                                std::initializer_list<T> hosts);
 
+  /**
+   * Adds a virtual camera for switching between two streams.  Unlike the
+   * other addCamera methods, this returns a VideoSink rather than a
+   * VideoSource.  Calling SetSource() on the returned object can be used
+   * to switch the actual source of the stream.
+   */
+  cs::MjpegServer AddSwitchedCamera(const wpi::Twine& name);
+
   /**
    * Get OpenCV access to the primary camera feed.  This allows you to
    * get images from the camera for image processing on the roboRIO.