Introduce Renderable.BeforeRender().

WorldRenderer.Draw() has been slightly reorganized
to ensure that BeforeRender is called before any
drawing render state (e.g. scissor) has been set.

OpenRA.Game/Graphics/VoxelRenderable.cs

@@ -79,6 +79,7 @@ namespace OpenRA.Graphics
 			                           palette, normalsPalette, shadowPalette);
 		}
 
+		public void BeforeRender(WorldRenderer wr) {}
 		public void Render(WorldRenderer wr)
 		{
 			// Depth and shadow buffers are cleared between actors so that