[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/linux/RunLoopHelpers.cpp

@@ -0,0 +1,13 @@
+// Copyright (c) FIRST and other WPILib contributors.
+// Open Source Software; you can modify and/or share it under the terms of
+// the WPILib BSD license file in the root directory of this project.
+
+#include "cscore_runloop.h"
+
+namespace cs {
+void RunOsxRunLoop() {}
+int RunOsxRunLoopTimeout(double timeoutSeconds) {
+  return 0;
+}
+void StopOsxMainRunLoop() {}
+}  // namespace cs