MjpegServer: Add ability to set compression, etc in code (#1229)

This allows code to set the stream compression, resolution, and FPS used if
not specified in HTTP parameters by the client.

cscore/src/main/native/cpp/MjpegServerImpl.h

@@ -55,6 +55,13 @@ class MjpegServerImpl : public SinkImpl {
   std::thread m_serverThread;
 
   std::vector<wpi::SafeThreadOwner<ConnThread>> m_connThreads;
+
+  // property indices
+  int m_widthProp;
+  int m_heightProp;
+  int m_compressionProp;
+  int m_defaultCompressionProp;
+  int m_fpsProp;
 };
 
 }  // namespace cs