[cscore] Add support for USB cameras on macOS (#4846)

The main restriction is there must be an event loop running on the main thread.
No special action is required for GUI applications, but for non-GUI applications, a
RunOsxRunLoop() function is provided that needs to be called from the main thread.

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

@@ -50,6 +50,7 @@ class SourceImpl : public PropertyContainer {
 
   void SetConnectionStrategy(CS_ConnectionStrategy strategy) {
     m_strategy = static_cast<int>(strategy);
+    NumSinksChanged();
   }
   bool IsEnabled() const {
     return m_strategy == CS_CONNECTION_KEEP_OPEN ||