input_common: Add support for GameCube Adapter

This is a port of the initial GameCube adapter input support i added into yuzu emulator.
It requires the same setup as when it was first introduced in yuzu, requiring the Zadig driver be installed for the adapter to allow it to interface with libusb.

src/input_common/gcadapter/gc_poller.h

@@ -0,0 +1,67 @@
+// Copyright 2020 yuzu Emulator Project
+// Licensed under GPLv2 or any later version
+// Refer to the license.txt file included.
+
+#pragma once
+
+#include <memory>
+#include "core/frontend/input.h"
+#include "input_common/gcadapter/gc_adapter.h"
+#include "input_common/main.h"
+
+namespace InputCommon {
+
+/**
+ * A button device factory representing a gcpad. It receives gcpad events and forward them
+ * to all button devices it created.
+ */
+class GCButtonFactory final : public Input::Factory<Input::ButtonDevice>,
+                              public Polling::DevicePoller {
+public:
+public:
+    explicit GCButtonFactory(std::shared_ptr<GCAdapter::Adapter> adapter_);
+
+    std::unique_ptr<Input::ButtonDevice> Create(const Common::ParamPackage& params) override;
+
+    Common::ParamPackage GetNextInput() override;
+
+    /// For device input configuration/polling
+    void Start() override;
+    void Stop() override;
+
+    bool IsPolling() const {
+        return polling;
+    }
+
+private:
+    std::shared_ptr<GCAdapter::Adapter> adapter;
+    bool polling{false};
+};
+
+/// An analog device factory that creates analog devices from GC Adapter
+class GCAnalogFactory final : public Input::Factory<Input::AnalogDevice>,
+                              public Polling::DevicePoller {
+public:
+    explicit GCAnalogFactory(std::shared_ptr<GCAdapter::Adapter> adapter_);
+
+    std::unique_ptr<Input::AnalogDevice> Create(const Common::ParamPackage& params) override;
+
+    Common::ParamPackage GetNextInput() override;
+
+    /// For device input configuration/polling
+    void Start() override;
+    void Stop() override;
+
+    bool IsPolling() const {
+        return polling;
+    }
+
+private:
+    std::shared_ptr<GCAdapter::Adapter> adapter;
+    int analog_x_axis{-1};
+    int analog_y_axis{-1};
+    int controller_number{-1};
+    bool polling{false};
+};
+
+} // namespace InputCommon