Export as ZIP: Pad frame numbers with leading zeroes

[andybarron]

Lack of leading zeroes causes a naive string sort to do this:
run_1, run_10, run_11, run_2, run_3, ...
Certain animation tools rely on the frame names to be zero-padded like so:
run_01, run_02, run_03, ... run_09, run_10, run_11, ...

[juliandescottes]

Thanks for filing!
Seems like a good first bug, so I'll drop a few hints if anyone wants to take it.

The export to ZIP is currently handled in https://github.com/juliandescottes/piskel/blob/master/src/js/controller/settings/exportimage/PngExportController.js

We currently set the zip file names in two methods : mergedExport_ and splittedExport_.
While implementing this, the duplicated code between these two methods could be removed.

The String pad method can be implemented in the PngExportController or in a /utils helper (either the existing core.js, or a new StringUtils.js file). A /utils helper would be a bit better, because we can easily unit test it.

[For information, the whole zip export should be moved out of the controller into a service, and the Zip export should have its own controller. But no need to address that here.]

The padding should be relative to the number of frames. E.g :

• if there are less than 10 frames, no padding
• more than 10 less than 100 : 01, 02, ..., 99.
• more than 100 less than 1000 : 001, 002, ... , 010, 011, ..., 999
• etc ... (don't think I ever saw a sprite made with piskel containing more than 1000 frames)

[juliandescottes]

Released in v0.7.0