Allow/require renderers to enable alpha blending when needed. Fixes voxel shadow issues.

OpenRA.Renderer.Null/NullGraphicsDevice.cs

@@ -41,6 +41,9 @@ namespace OpenRA.Renderer.Null
 		public void EnableDepthBuffer() { }
 		public void DisableDepthBuffer() { }
 
+		public void EnableAlphaBlending() { }
+		public void DisableAlphaBlending() { }
+
 		public void Clear() { }
 		public void Present() { }