[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/UsbCameraDelegate.h

@@ -0,0 +1,22 @@
+// 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.
+
+#pragma once
+
+#import <AVFoundation/AVFoundation.h>
+#include <memory>
+
+namespace cs {
+class UsbCameraImpl;
+}
+
+@interface UsbCameraDelegate
+    : NSObject <AVCaptureVideoDataOutputSampleBufferDelegate>
+
+@property(nonatomic) std::weak_ptr<cs::UsbCameraImpl> cppImpl;
+
+- (void)captureOutput:(AVCaptureOutput*)captureOutput
+    didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer
+           fromConnection:(AVCaptureConnection*)connection;
+@end