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.Game/Sound.cs

@@ -607,7 +607,7 @@ namespace OpenRA
 			slot.FrameStarted = currFrame;
 			slot.Sound = sound;
 			slot.IsRelative = relative;
-			return new OpenAlSound(source, (sound as OpenAlSoundSource).Buffer, loop, relative, pos, volume * atten);
+			return new OpenAlSound(source, ((OpenAlSoundSource)sound).Buffer, loop, relative, pos, volume * atten);
 		}
 
 		public float Volume