Add glDrawElements

OpenRA.Game/Graphics/PlatformInterfaces.cs

@@ -94,6 +94,7 @@ namespace OpenRA
 		void DisableScissor();
 		void Present();
 		void DrawPrimitives(PrimitiveType pt, int firstVertex, int numVertices);
+		void DrawElements(int numIndices, int offset);
 		void Clear();
 		void EnableDepthBuffer();
 		void DisableDepthBuffer();