General uncontroversial cleanup:

- Made private methods static where possible (runtime can elide checking the object for null).
- Declared attribute classes as sealed (allows reflection on attributes to complete faster).
- Moved some static cctor's into field initializers (static cctor's are slower than static field initializers).
- Made classes static if they contained only static methods (can't create instances of useless objects).
- Use inferable Exts.Lazy and not new Lazy<T>().
- Added required STAThread attribute to CrashDialog.
- Removed unused parameters in private methods.
- Added Serializable attribute to exceptions.
- Added parameter name in calls to ArgumentNullException.
- Use of as operator instead of is + cast.
- Changed (x as Foo).Bar anti-pattern into ((Foo)x).Bar. Results in sensible cast exceptions on error rather than null dereferences.
- Removed unused method in NullShader.

OpenRA.Renderer.Sdl2/Sdl2Input.cs

@@ -17,7 +17,7 @@ namespace OpenRA.Renderer.Sdl2
 	{
 		MouseButton lastButtonBits = (MouseButton)0;
 
-		MouseButton MakeButton(byte b)
+		static MouseButton MakeButton(byte b)
 		{
 			return b == SDL.SDL_BUTTON_LEFT ? MouseButton.Left
 				: b == SDL.SDL_BUTTON_RIGHT ? MouseButton.Right
@@ -25,7 +25,7 @@ namespace OpenRA.Renderer.Sdl2
 				: 0;
 		}
 
-		Modifiers MakeModifiers(int raw)
+		static Modifiers MakeModifiers(int raw)
 		{
 			return ((raw & (int)SDL.SDL_Keymod.KMOD_ALT) != 0 ? Modifiers.Alt : 0)
 				 | ((raw & (int)SDL.SDL_Keymod.KMOD_CTRL) != 0 ? Modifiers.Ctrl : 0)