Dispose of graphics resources deterministically.

Textures, FrameBuffers and VertexBuffers allocated by the Sdl2 Renderer were only being released via finalizers. This could lead to OpenGL out of memory errors since resources may not be cleaned up in a timely manner. To avoid this, IDisposable has been implemented and transitively applied to classes that use these resources.

As a side-effect some static state is no longer static, particularly in Renderer, in order to facilitate this change and just for nicer design in general.

Also dispose some bitmaps.

OpenRA.Renderer.Null/NullGraphicsDevice.cs

@@ -78,7 +78,7 @@ namespace OpenRA.Renderer.Null
 		public void Render(Action a) { }
 	}
 
-	public class NullTexture : ITexture
+	public sealed class NullTexture : ITexture
 	{
 		public TextureScaleFilter ScaleFilter { get { return TextureScaleFilter.Nearest; } set { } }
 		public void SetData(Bitmap bitmap) { }
@@ -86,18 +86,21 @@ namespace OpenRA.Renderer.Null
 		public void SetData(byte[] colors, int width, int height) { }
 		public byte[] GetData() { return new byte[0]; }
 		public Size Size { get { return new Size(0, 0); } }
+		public void Dispose() { }
 	}
 
-	public class NullFrameBuffer : IFrameBuffer
+	public sealed class NullFrameBuffer : IFrameBuffer
 	{
 		public void Bind() { }
 		public void Unbind() { }
 		public ITexture Texture { get { return new NullTexture(); } }
+		public void Dispose() { }
 	}
 
-	class NullVertexBuffer<T> : IVertexBuffer<T>
+	sealed class NullVertexBuffer<T> : IVertexBuffer<T>
 	{
 		public void Bind() { }
 		public void SetData(T[] vertices, int length) { }
+		public void Dispose() { }
 	}
 }