First attempt at rendering polyobject previews.

Source/Core/Rendering/IRenderer2D.cs

@@ -68,6 +68,8 @@ namespace CodeImp.DoomBuilder.Rendering
 		void PlotVerticesSet(ICollection<Vertex> vertices);
 		void RenderThing(Thing t, PixelColor c, float alpha);
 		void RenderThingSet(ICollection<Thing> things, float alpha);
+		void RenderWaypoints();
+		void RenderPolyobjects();
         void RenderNiGHTSPath();
         void RenderRectangle(RectangleF rect, float bordersize, PixelColor c, bool transformrect);
 		void RenderRectangleFilled(RectangleF rect, PixelColor c, bool transformrect);