WindowServer: Create the VirtualScreenBackend

This screen backend is just memory-backed and doesn't connect to any
screen hardware. That way, we can boot Serenity without video hardware
but in full graphical mode :^)

To create a virtual screen, put something like this in your
WindowServer.ini. There's no way yet to do this through Display
Settings, though an existing virtual screen's settings can be changed
there.
```ini
[Screen0]
Mode=Virtual
Left=1024
Top=0
Width=1920
Height=1080
ScaleFactor=1
```

Userland/Services/WindowServer/VirtualScreenBackend.h

@@ -0,0 +1,44 @@
+/*
+ * Copyright (c) 2022, kleines Filmröllchen <filmroellchen@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include "ScreenBackend.h"
+#include "ScreenLayout.h"
+#include <AK/Error.h>
+#include <AK/Span.h>
+#include <AK/String.h>
+#include <sys/ioctl_numbers.h>
+
+namespace WindowServer {
+
+class VirtualScreenBackend : public ScreenBackend {
+public:
+    virtual ~VirtualScreenBackend();
+
+    VirtualScreenBackend() = default;
+
+private:
+    friend class Screen;
+
+    virtual ErrorOr<void> open() override;
+
+    virtual void set_head_buffer(int index) override;
+
+    virtual ErrorOr<void> flush_framebuffer_rects(int, Span<FBRect const>) override { return {}; }
+
+    virtual ErrorOr<void> unmap_framebuffer() override;
+    virtual ErrorOr<void> map_framebuffer() override;
+
+    virtual ErrorOr<void> set_head_resolution(FBHeadResolution) override;
+    virtual ErrorOr<FBHeadProperties> get_head_properties() override;
+
+    int m_height { 0 };
+    int m_width { 0 };
+    bool m_first_buffer_active { true };
+};
+
+}