Let Texture Packer pack pages according to setup pose draw order.

There's an issue when a user wants to use very-high-resolution assets so the parts don't fit into one texture.

If the regions aren't grouped properly, the parts can alternate wildly between two or more textures.
This is what it looks like in Unity. Each entry in the list is one draw call, just for this one character.

In most runtimes, this leads to bad batching. In Unity's case, it causes the renderer to do excess work, and also cause weird sorting issues with a perspective camera that needs move closer and farther away from the rendered object.