Use a single libusb context

libusb on Windows is limited to only a single context. Trying to open
more than one can cause device enumerations to fail randomly.

libusb is thread-safe and we don't use the manual polling support (with
`poll()`) so this should be safe.

Source/Core/Common/LibusbContext.cpp

@@ -0,0 +1,46 @@
+// Copyright 2017 Dolphin Emulator Project
+// Licensed under GPLv2+
+// Refer to the license.txt file included.
+
+#include <libusb.h>
+#include <memory>
+#include <mutex>
+
+#include "Common/LibusbContext.h"
+#include "Common/MsgHandler.h"
+
+namespace LibusbContext
+{
+static std::shared_ptr<libusb_context> s_libusb_context;
+static std::once_flag s_context_initialized;
+
+static libusb_context* Create()
+{
+  libusb_context* context;
+  const int ret = libusb_init(&context);
+  if (ret < LIBUSB_SUCCESS)
+  {
+    bool is_windows = false;
+#ifdef _WIN32
+    is_windows = true;
+#endif
+    if (is_windows && ret == LIBUSB_ERROR_NOT_FOUND)
+      PanicAlertT("Failed to initialize libusb because usbdk is not installed.");
+    else
+      PanicAlertT("Failed to initialize libusb: %s", libusb_error_name(ret));
+    return nullptr;
+  }
+  return context;
+}
+
+std::shared_ptr<libusb_context> Get()
+{
+  std::call_once(s_context_initialized, []() {
+    s_libusb_context.reset(Create(), [](auto* context) {
+      if (context != nullptr)
+        libusb_exit(context);
+    });
+  });
+  return s_libusb_context;
+}
+}