Add .dds support and update manual.

Author: Csantucci

Source/Documentation/Manual/features-rollingstock.rst

@@ -344,11 +344,14 @@ Detailed spec
 1) In the ``Content`` folder there is the default ``LightGlow.png``, which is displayed if 
    no changes are done to the .eng file.
 2) In such folder there is also an ``ORTSLightGlow.png``, which is maybe more realistic.
-3) adding a line within the .eng file it is possible to select either ORTSLightGlow.png or any other .png. 
+3) adding a line within the .eng file it is possible to select either ORTSLightGlow.png or any other picture
+   with extension ``.png, .jpg, bmp, .gif, .ace, or .dds``.
+   
+
    Here an example for the legacy Acela loco::
 
     	Lights	( 17
-	        ORTSGraphic ( "ORTSLightGlow.png")
+	        ORTSGraphic ( "ORTSLightGlow.png" )
 		      Light	(
 			      comment( Sphere of light )
 			      Type	( 1 )
@@ -357,7 +360,7 @@ Detailed spec
   The code first searches for the .png file by building its directory starting from the directory of 
   the .eng file; in this case the line could be e.g.::
 
-           ORTSGraphic ( "ORTSAcelaLightGlow.png")
+           ORTSGraphic ( "ORTSAcelaLightGlow.png" )
 
 4) The ``ORTSGraphic`` line can be added also for one or more ``Light()`` blocks. In that case the 
    .png file is used only for the related Light block. Here an example::

Source/RunActivity/Viewer3D/Materials.cs

@@ -24,6 +24,7 @@
 using System.IO;
 using System.Linq;
 using System.Threading;
+using Microsoft.CodeAnalysis.VisualBasic.Syntax;
 using Microsoft.Xna.Framework;
 using Microsoft.Xna.Framework.Graphics;
 using Orts.Viewer3D.Common;
@@ -179,7 +180,13 @@ public static Texture2D Get(GraphicsDevice graphicsDevice, string path)
 
             if (ext == ".ace")
                 return Orts.Formats.Msts.AceFile.Texture2DFromFile(graphicsDevice, path);
-
+            else if (ext == ".dds" && File.Exists(path))
+            {
+                Texture2D ddsTexture;
+                DDSLib.DDSFromFile(path, graphicsDevice, true, out ddsTexture);
+                return ddsTexture;
+            }
+                    
             using (var stream = File.OpenRead(path))
             {
                 if (ext == ".gif" || ext == ".jpg" || ext == ".png")