Add some missing Dispose calls, fix some Dispose implementations.

OpenRA.Game/Graphics/SpriteFont.cs

@@ -17,7 +17,7 @@ using SharpFont;
 
 namespace OpenRA.Graphics
 {
-	public class SpriteFont
+	public sealed class SpriteFont : IDisposable
 	{
 		static readonly Library Library = new Library();
 
@@ -150,6 +150,11 @@ namespace OpenRA.Graphics
 
 			return g;
 		}
+
+		public void Dispose()
+		{
+			face.Dispose();
+		}
 	}
 
 	class GlyphInfo