[Clipart] Making our thumbnails comply with the thumbnail spec, i.e., 128x128px

[Jonadab the Unsightly One]

At some point (not for this release), we will want to make the
thumbnails in our packages usable by apps that use the fdo thumbnail
specification for thumbnails.  The (theoretical, currently
non-existent -- thus, no hurry) unpackager/installer will handle
storing them in the right location on the filesystem according to the
spec, but it can avoid having to parse any SVG or generate any
thumbnails itself if the thumbnails we provide are usable.  We already
use PNG, which is the format the spec sanctions, so we're partway
there, but...  they will have to be square, 128x128 pixels (for
"normal" size, which is the smallest size specified in the spec).  Of
course, many of the images aren't anywhere near square in shape, and
stretching them in a way that distorts the aspect ratio seems to me
like the wrong approach.

So, here's a thread where we can discuss what the right approach is.
Cropping to a square or centering (on a square background) are the
other obvious approaches.  Is one of these less evil than the other?
Would a combination of the two be a good compromise, or the worst of
both worlds?  Discuss.

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