set a default delay if delay is 0 (#445)

gif spec [0] doesn't mention what to do when "Delay Time" is 0.

apng spec [1] states:

| If the the value of the numerator is 0 the decoder should render the
| next frame as quickly as possible, though viewers may impose a
| reasonable lower bound.

webp spec [2]:

| the interpretation of frame duration of 0 (and often <= 10) is
| implementation defined.

so it seems that it's safe to set a default delay for 0 delay frames,
which is what the older gif and webp loaders were already doing. do the
same for the imlib2 multi-frame loader as well.

[0]: https://www.w3.org/Graphics/GIF/spec-gif89a.txt
[1]: https://wiki.mozilla.org/APNG_Specification
[2]: https://developers.google.com/speed/webp/docs/riff_container#animation

Reviewed-on: https://codeberg.org/nsxiv/nsxiv/pulls/445
Reviewed-by: eylles <eylles@noreply.codeberg.org>

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