Fix indexed color type images

Author: fidifis

TextureMerge/Merge/Merge.cs

@@ -363,6 +363,7 @@ public void LoadChannel(string path, Channel channelSlot, Channel channelSource)
 
 
             var source = new TMImage(new MagickImage(path), Path.GetFileName(path));
+            source.Image.ColorType = ColorType.TrueColor;
             source.Image.AutoOrient();
             source.Image.ColorSpace = ColorSpace.sRGB;