GRAPHICS: Be more robust with broken/unsupported thumbnail headers.

This fixes future issues like bug #3614654:
"ALL: ScummVM 1.5.0 can't read newer saved games".

There are a few behavior changes introduced with this commit:

- checkThumbnailHeader will now also report the presence of
  unsupported/broken (but skippable) headers.

- skipThumbnail will also try to skip the data for broken/unsupported
  thumbnail data.

- loadThumbnail will skip over broken/unsupported thumbnail data but still
  return 0 in this case.