[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.mm

@@ -0,0 +1,67 @@
+// 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.
+
+#import "UsbCameraDelegate.h"
+#include "UsbCameraImpl.h"
+
+#include <wpi/timestamp.h>
+
+#include <opencv2/core/core.hpp>
+#include <opencv2/imgproc/imgproc.hpp>
+
+@implementation UsbCameraDelegate
+
+- (id)init {
+  self = [super init];
+  return self;
+}
+
+- (void)captureOutput:(AVCaptureOutput*)captureOutput
+    didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer
+           fromConnection:(AVCaptureConnection*)connection {
+  (void)captureOutput;
+  (void)sampleBuffer;
+  (void)connection;
+
+  auto sharedThis = self.cppImpl.lock();
+  if (!sharedThis) {
+    return;
+  }
+
+  // Buffer always comes in a 32BGRA
+  auto imageBuffer = CMSampleBufferGetImageBuffer(sampleBuffer);
+
+  CVPixelBufferLockBaseAddress(imageBuffer, 0);
+
+  void* baseaddress = CVPixelBufferGetBaseAddress(imageBuffer);
+
+  size_t width = CVPixelBufferGetWidth(imageBuffer);
+  size_t height = CVPixelBufferGetHeight(imageBuffer);
+  size_t rowBytes = CVPixelBufferGetBytesPerRow(imageBuffer);
+  OSType pixelFormat = CVPixelBufferGetPixelFormatType(imageBuffer);
+
+  if (rowBytes == 0) {
+    CVPixelBufferUnlockBaseAddress(imageBuffer, 0);
+    return;
+  }
+
+  if (pixelFormat != kCVPixelFormatType_32BGRA) {
+    NSLog(@"Unknown Pixel Format %u", pixelFormat);
+    CVPixelBufferUnlockBaseAddress(imageBuffer, 0);
+    return;
+  }
+
+  size_t currSize = width * 3 * height;
+
+  auto tmpMat = cv::Mat(height, width, CV_8UC4, baseaddress, rowBytes);
+  auto image = sharedThis->AllocImage(cs::VideoMode::PixelFormat::kBGR, width,
+                                      height, currSize);
+  cv::cvtColor(tmpMat, image->AsMat(), cv::COLOR_BGRA2BGR);
+
+  CVPixelBufferUnlockBaseAddress(imageBuffer, 0);
+
+  sharedThis->objcPutFrame(std::move(image), wpi::Now());
+}
+
+@end