[1/2] windowscodecs: Implement GetPreview and GetThumbnail in the TIFF decoder.

[Vincent Povirk]

According to MSDN, GetPreview isn't supported on any decoder except
WMPhoto, and if that's true it should probably be returning
WINCODEC_ERR_UNSUPPORTEDOPERATION.

And why bother with GetThumbnail if you're not going to do it
properly? At least check whether a thumbnail exists in the image
before returning something.