Add GetNextFrame timeout to CvSink and MjpegServer.

MjpegServer uses the timeout to generate keep-alives to any clients
(which helps detect disconnects and avoid stale client threads).

CvSink GrabFrame now defaults to a timeout, but the timeout can be
changed by the user, or the old no-timeout version is now available
as GrabFrameNoTimeout.

java/lib/CameraServerJNI.cpp

@@ -1261,6 +1261,21 @@ JNIEXPORT jlong JNICALL Java_edu_wpi_cscore_CameraServerJNI_grabSinkFrame
   return rv;
 }
 
+/*
+ * Class:     edu_wpi_cscore_CameraServerJNI
+ * Method:    grabSinkFrameTimeout
+ * Signature: (IJD)J
+ */
+JNIEXPORT jlong JNICALL Java_edu_wpi_cscore_CameraServerJNI_grabSinkFrameTimeout
+  (JNIEnv *env, jclass, jint sink, jlong imageNativeObj, jdouble timeout)
+{
+  cv::Mat& image = *((cv::Mat*)imageNativeObj);
+  CS_Status status = 0;
+  auto rv = cs::GrabSinkFrameTimeout(sink, image, timeout, &status);
+  CheckStatus(env, status);
+  return rv;
+}
+
 /*
  * Class:     edu_wpi_cscore_CameraServerJNI
  * Method:    getSinkError

java/src/edu/wpi/cscore/CameraServerJNI.java

@@ -193,6 +193,7 @@ public class CameraServerJNI {
   //
   public static native void setSinkDescription(int sink, String description);
   public static native long grabSinkFrame(int sink, long imageNativeObj);
+  public static native long grabSinkFrameTimeout(int sink, long imageNativeObj, double timeout);
   public static native String getSinkError(int sink);
   public static native void setSinkEnabled(int sink, boolean enabled);
 

java/src/edu/wpi/cscore/CvSink.java

@@ -39,10 +39,28 @@ public class CvSink extends VideoSink {
   }
 
   /// Wait for the next frame and get the image.
+  /// Times out (returning 0) after 0.225 seconds.
   /// The provided image will have three 3-bit channels stored in BGR order.
   /// @return Frame time, or 0 on error (call GetError() to obtain the error
   ///         message);
   public long grabFrame(Mat image) {
+    return grabFrame(image, 0.225);
+  }
+
+  /// Wait for the next frame and get the image.
+  /// Times out (returning 0) after timeout seconds.
+  /// The provided image will have three 3-bit channels stored in BGR order.
+  /// @return Frame time, or 0 on error (call GetError() to obtain the error
+  ///         message);
+  public long grabFrame(Mat image, double timeout) {
+    return CameraServerJNI.grabSinkFrameTimeout(m_handle, image.nativeObj, timeout);
+  }
+
+  /// Wait for the next frame and get the image.  May block forever.
+  /// The provided image will have three 3-bit channels stored in BGR order.
+  /// @return Frame time, or 0 on error (call GetError() to obtain the error
+  ///         message);
+  public long grabFrameNoTimeout(Mat image) {
     return CameraServerJNI.grabSinkFrame(m_handle, image.nativeObj);
   }