[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/objcpp/RunLoopHelpers.mm

@@ -0,0 +1,21 @@
+// 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"
+
+#include <CoreFoundation/CFRunLoop.h>
+
+namespace cs {
+void RunOsxRunLoop() {
+  CFRunLoopRun();
+}
+
+int RunOsxRunLoopTimeout(double timeoutSeconds) {
+  return CFRunLoopRunInMode(kCFRunLoopDefaultMode, timeoutSeconds, false);
+}
+
+void StopOsxMainRunLoop() {
+  CFRunLoopStop(CFRunLoopGetMain());
+}
+}