WindowServer: Implement fullscreen windows

If window size does not match framebuffer size, window data will be
scaled to framebuffer using nearest sampling for best performance.

userspace/programs/WindowServer/main.cpp

@@ -371,6 +371,10 @@ int main()
 						if (auto ret = LibGUI::WindowPacket::WindowSetSize::deserialize(client_data.packet_buffer.span()); !ret.is_error())
 							window_server.on_window_set_size(fd, ret.release_value());
 						break;
+					case LibGUI::PacketType::WindowSetFullscreen:
+						if (auto ret = LibGUI::WindowPacket::WindowSetFullscreen::deserialize(client_data.packet_buffer.span()); !ret.is_error())
+							window_server.on_window_set_fullscreen(fd, ret.release_value());
+						break;
 					default:
 						dprintln("unhandled packet type: {}", *reinterpret_cast<uint32_t*>(client_data.packet_buffer.data()));
 				}