showThumbnail (filename, x, y);
Retrieves the thumbnail image from a saved game file (filename) and displays the image on the screen at the co-ordinates x, y (top left corner).
If filename is a valid saved game which contains no thumbnail image, the code has no effect. If the file does not exist your game will exit with the appropriate fatal error - therefore it's a good idea to check whether the fileExists first.
This function supports filename bodging - for details see the bodgeFilenames command.
No return value.
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