[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/UnlimitedHandleResource.h

@@ -154,7 +154,7 @@ template <typename T>
 inline std::span<T>
 UnlimitedHandleResource<THandle, TStruct, typeValue, TMutex>::GetAll(
     wpi::SmallVectorImpl<T>& vec) {
-  ForEach([&](THandle handle, const TStruct& data) { vec.push_back(handle); });
+  ForEach([&](THandle handle, const TStruct&) { vec.push_back(handle); });
   return vec;
 }