The changes to the sorting rules caused a breaking change that was incompatible.
The solution to this is to revert to being compatible with <=2020.3 but also to allow modellers to choose the new sorting rules.
I've added a new tag <defaults> which can contain either <axis-animation-vertex-order-xyz/> or <axis-animation-vertex-order-x/>. The defaults will affect all animations in the .xml and also any included models unless the included model also has a <defaults> section.
So for modelers that want to use the new definitive vertex sorting rules adding the following to the main model is sufficient.
<defaults>
<axis-animation-vertex-order-xyz/>
</defaults>
There are also a couple of new tags that can be used in the <axis> section.
1. <order-by-xyz/> - use the new sorting rules
2. <order-by-x/> - use the 2020.3 sorting rules
3. <swap-axis-direction/> - when the animation goes the wrong way this is an elegant way to fix it.
See https://sourceforge.net/p/flightgear/codetickets/2706/
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