This PR adds a commandline option -headless which runs the game without any graphics.
This is necessary to implement a silent replay checker (see issue #565). Note that I have that replay checker already implemented, but I will push that in another PR. This is just preparation for that.

Note that right now, if you pass in -headless, the game just runs the Shellmap. Since it's headless, you won't be able to see or do anything. But if you run it in Debug, you can enjoy looking at the log file getting bigger.

Headless mode is implemented here by essentially only running GameLogic, and not the GameClient. Unfortunately, GameLogic depends on the GameClient in some small hacky ways, which makes this a non-trivial task. See also this comment I added:

// TheSuperHackers @logic-client-separation helmutbuhler 04/11/2025
// Some information about the architecture and headless mode:
// The game is structurally separated into GameLogic and GameClient.
// The Logic is responsible for everything that affects the game mechanic and what is synchronized over
// the network. The Client is responsible for rendering, input, audio and similar stuff.
// 
// Unfortunately there are some places in the code that make the Logic depend on the Client.
// (Search for @logic-client-separation)
// That means if we want to run the game headless, we cannot just disable the Client. We need to disable
// the parts in the Client that don't work in headless mode and need to keep the parts that are needed
// to run the Logic.
// The following describes which parts we disable in headless mode:
//
//	GameEngine:
//		TheGameClient is partially disabled:
//			TheKeyboard = NULL
//			TheMouse = NULL
//			TheDisplay is partially disabled:
//				m_3DInterfaceScene = NULL
//				m_2DScene = NULL
//				m_3DScene = NULL
//				(m_assetManager remains!)
//			TheWindowManager = NULL
//			TheIMEManager = NULL
//			TheTerrainVisual is partially disabled:
//				TheTerrainTracksRenderObjClassSystem = NULL
//				TheW3DShadowManager = NULL
//				TheWaterRenderObj = NULL
//				TheSmudgeManager = NULL
//				TheTerrainRenderObject is partially disabled:
//					m_treeBuffer = NULL
//					m_propBuffer = NULL
//					m_bibBuffer = NULL
//					m_bridgeBuffer = NULL
//					m_waypointBuffer = NULL
//					m_roadBuffer = NULL
//					m_shroud = NULL
//		TheRadar = RadarDummy

As it is now, this PR sets all the client components as listed above to NULL and adds all the appropriate NULL-checks (which are sadly quite a few).
For TheRadar I added a dummy implementation to avoid the NULL-checks in the many callers (this doesn't make much sense in the other components though).

If we decide to fix these separation issues properly in the future, we will be able to revert many of the changes in this PR in the future.

Alternatively, we can also decide to just hide the window in headless mode and initialize DX without using it. Then almost all the changes in this PR become obsolete as well. The question is then whether this will still run in CI.

I would prefer to keep the changes in this PR and to clean up these separation issues properly long-term. Having logic and client properly separated can help in the future when we decide to increase the framerate of the client, or if we want to implement a headless server. It's also nice to automatically test that the logic works without the client.

TODO

• Replicate in Generals