viewflif: always render the last callback

[hrj]

When viewing very large FLIFs, the final callback may not be at quality = 100%, because of scaling.

This change ensures that the last callback is rendered.

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Addendum:

• When options.fit == true, fit the preview images as well
• Disabled transform-before-interpolation when scale != 1, as it is not stable for some images

Closes #333

[hrj]

I fixed a few more things:

• When options.fit == true, fit the preview images as well
• Disabled transform-before-interpolation when scale != 1, as it is not stable for some images