Add display.max_fps CVar.

Applies when vsync is not enabled.

Had to shuffle stuff around to GameController since it involves the game loop.

The implementation isn't great and undershoots the target FPS value (because the OS overshoots the desired sleep value). I tried using SDL_DelayPrecise too but this causes significantly increased CPU usage probably because it spinwaits and all that nonsense, so I decided against it.

I don't know why I bothered to do this. I just got the idea in my head. Kinda feels like a waste of time, but there's no point not committing it at this point.

Robust.Client/GameController/GameController.cs

@@ -110,6 +110,8 @@ namespace Robust.Client
 
         private ResourceManifestData? _resourceManifest;
 
+        private DisplayMode _displayMode;
+
         public void SetCommandLineArgs(CommandLineArgs args)
         {
             _commandLineArgs = args;
@@ -273,6 +275,9 @@ namespace Robust.Client
                 }
             };
 
+            _configurationManager.OnValueChanged(CVars.DisplayMaxFPS, _ => UpdateVsyncConfig());
+            _configurationManager.OnValueChanged(CVars.DisplayVSync, _ => UpdateVsyncConfig(), invokeImmediately: true);
+
             _clyde.Ready();
 
             if (_resourceManifest!.AutoConnect &&
@@ -709,6 +714,30 @@ namespace Robust.Client
         }
 
 
+        private void UpdateVsyncConfig()
+        {
+            if (_displayMode == DisplayMode.Headless)
+                return;
+
+            var vsync = _configurationManager.GetCVar(CVars.DisplayVSync);
+            var maxFps = Math.Clamp(_configurationManager.GetCVar(CVars.DisplayMaxFPS), 0, 10_000);
+
+            _clyde.VsyncEnabled = vsync;
+
+            if (_mainLoop == null)
+                return;
+
+            if (vsync || maxFps == 0)
+            {
+                _mainLoop.SleepMode = SleepMode.None;
+            }
+            else
+            {
+                _mainLoop.SleepMode = SleepMode.Limit;
+                _mainLoop.LimitMinFrameTime = TimeSpan.FromSeconds(1.0 / maxFps);
+            }
+        }
+
         internal enum DisplayMode : byte
         {
             Headless,