Fix Image loading from file with mono 4.9

Apply dbe0dfe for all platforms.
Mono uses referencesource impl since mono/mono#4042, which does
not allow pathed search patterns in directory file lookups.

Xwt/Xwt.Drawing/Image.cs

@@ -1054,9 +1054,7 @@ public override IEnumerable<string> GetAlternativeFiles (string fileName, string
 						yield return fn;
 				}
 			} else {
-				if (Path.DirectorySeparatorChar == '\\') // windows)
-					baseName = Path.GetFileName (baseName);
-				var files = Directory.GetFiles (Path.GetDirectoryName (fileName), baseName + "*" + ext);
+				var files = Directory.GetFiles (Path.GetDirectoryName (fileName), Path.GetFileName (baseName) + "*" + ext);
 				foreach (var f in files)
 					yield return f;
 			}