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.cpp

@@ -0,0 +1,81 @@
+/*
+ * Copyright (c) 2018-2020, the SerenityOS developers.
+ * Copyright (c) 2022, kleines Filmröllchen <filmroellchen@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include "VirtualScreenBackend.h"
+#include "ScreenBackend.h"
+#include <AK/Try.h>
+#include <AK/kmalloc.h>
+#include <LibGfx/Color.h>
+
+namespace WindowServer {
+
+VirtualScreenBackend::~VirtualScreenBackend()
+{
+    MUST(unmap_framebuffer());
+}
+
+ErrorOr<void> VirtualScreenBackend::open()
+{
+    m_can_device_flush_buffers = true;
+    m_can_set_head_buffer = true;
+    return {};
+}
+
+void VirtualScreenBackend::set_head_buffer(int index)
+{
+    VERIFY(index <= 1 && index >= 0);
+    m_first_buffer_active = index == 0;
+}
+
+ErrorOr<void> VirtualScreenBackend::set_head_resolution(FBHeadResolution resolution)
+{
+    if (resolution.head_index != 0)
+        return Error::from_string_literal("Only head index 0 is supported."sv);
+    m_height = resolution.height;
+
+    if (resolution.pitch == 0)
+        resolution.pitch = static_cast<int>(resolution.width * sizeof(Gfx::ARGB32));
+    m_pitch = resolution.pitch;
+    if (static_cast<int>(resolution.width * sizeof(Gfx::ARGB32)) != resolution.pitch)
+        return Error::from_string_literal("Unsupported pitch"sv);
+
+    m_width = resolution.width;
+    return {};
+}
+
+ErrorOr<FBHeadProperties> VirtualScreenBackend::get_head_properties()
+{
+    return FBHeadProperties {
+        .head_index = 0,
+        .pitch = static_cast<unsigned>(m_pitch),
+        .width = static_cast<unsigned>(m_width),
+        .height = static_cast<unsigned>(m_height),
+        .offset = static_cast<unsigned>(m_first_buffer_active ? 0 : m_back_buffer_offset),
+        .buffer_length = static_cast<unsigned>(m_size_in_bytes),
+    };
+}
+
+ErrorOr<void> VirtualScreenBackend::map_framebuffer()
+{
+    m_size_in_bytes = static_cast<unsigned long>(m_pitch) * m_height * 2;
+    m_framebuffer = new Gfx::ARGB32[static_cast<unsigned long>(m_width) * m_height * 2];
+    if (m_framebuffer == nullptr)
+        return Error::from_errno(ENOMEM);
+
+    m_back_buffer_offset = m_size_in_bytes / 2;
+    m_first_buffer_active = true;
+
+    return {};
+}
+
+ErrorOr<void> VirtualScreenBackend::unmap_framebuffer()
+{
+    delete[] m_framebuffer;
+    return {};
+}
+
+}