Respect slice keys frame number on ase file format (fix #4062)

[npinochet]

When saving slices on the ASE file format, the starting frame of the slice was not respected and always try to save the slice on frame 0 (fromFrame). Now it takes the fromFrame from the slice if the keys are not empty.
Fixes #4062.

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I have signed the CLA following the steps given in https://github.com/igarastudio/cla#signing

[dacap]

Hi @npinochet, thanks for this PR, remember to sign the CLA document you've received via email from signNow as a last step.

[npinochet]

Hi @npinochet, thanks for this PR, remember to sign the CLA document you've received via email from signNow as a last step.

Oh I totally missed that email, now it's signed. Thanks!

[dacap]

@npinochet sorry, I was going to review this PR but saw that the CLA was signed with a different email address that the one used in this commit (or the email wasn't included in the list of emails)

[npinochet]

@npinochet sorry, I was going to review this PR but saw that the CLA was signed with a different email address that the one used in this commit (or the email wasn't included in the list of emails)

Oh I'm sorry, I haven't updated my email on git for a long time, now the emails should match

[dacap]

Thanks for the fix @npinochet, it works great 👍

I'm going to merge this as it is, but there are some extreme cases that might be broken, e.g. using the CLI with --frame-range. But I'll add some unit tests to check and fix this with following commits.