Add some missing Dispose calls, fix some Dispose implementations.

OpenRA.Game/Graphics/HardwareCursor.cs

@@ -17,7 +17,7 @@ using OpenRA.Primitives;
 
 namespace OpenRA.Graphics
 {
-	public class HardwareCursor : ICursor
+	public sealed class HardwareCursor : ICursor
 	{
 		readonly Dictionary<string, IHardwareCursor[]> hardwareCursors = new Dictionary<string, IHardwareCursor[]>();
 		readonly CursorProvider cursorProvider;

OpenRA.Game/Graphics/SoftwareCursor.cs

@@ -24,7 +24,7 @@ namespace OpenRA.Graphics
 		void Tick();
 	}
 
-	public class SoftwareCursor : ICursor
+	public sealed class SoftwareCursor : ICursor
 	{
 		readonly HardwarePalette palette = new HardwarePalette();
 		readonly Cache<string, PaletteReference> paletteReferences;

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