Allow reordering PNG slice dimensions

[xingyzt]

Currently, the PNG Slices export slices along the Z axis and lays the planes down along the X axis. This may create slices that are undesirably large in the X axis if the scene has a high X:Y aspect ratio. I'm not very familiar with C, especially making interfaces with it, but I have made a simple edit that lets one change the axes with two numerical inputs of the range 0-2.

Prototype GUI

Z slicing axis, X laying axis: the default, which may be too long for image editors like ImageMagick who has a 10k limit per dimension.

Z slicing axis, Y laying axis: done by setting the laying axis to 1 (Y) instead of 0 (X):

[guillaumechereau]

Hello FlyOrBoom. Before I check the PR, can you make a new PR that adds yourself to the list of CLA, as explained here: https://github.com/guillaumechereau/goxel/blob/master/CONTRIBUTING.md

I need that to keep the right to license goxel for iOS.