cscore: Add Shutdown() function

This allows ordered shutdown / destruction of sources and sinks prior to
global singleton destruction.

cscore/src/main/native/cpp/jni/CameraServerJNI.cpp

@@ -87,6 +87,8 @@ JNIEXPORT jint JNICALL JNI_OnLoad(JavaVM* vm, void* reserved) {
 }
 
 JNIEXPORT void JNICALL JNI_OnUnload(JavaVM* vm, void* reserved) {
+  cs::Shutdown();
+
   JNIEnv* env;
   if (vm->GetEnv(reinterpret_cast<void**>(&env), JNI_VERSION_1_6) != JNI_OK)
     return;