The code added in #14343 seems to be trying to ensure that a `Handle`
for each glTF node exists by topologically sorting the directed graph of
glTF nodes containing edges from parent to child and from skin to joint.
Unfortunately, such a graph can contain cycles, as there's no guarantee
that joints are descendants of nodes with the skin. In particular, glTF
exported from Maya using the popular babylon.js export plugin create
skins attached to nodes that animate their parent nodes. This was
causing the topological sort code to enter an infinite loop.
Assuming that the intent of the topological sort is indeed to ensure
that `Handle`s exist for each glTF node before populating them, there's
a better mechanism for this: `LoadContext::get_label_handle`. This is
the documented way to obtain a handle for a node before populating it,
obviating the need for a topological sort. This patch replaces the
topological sort with a pre-pass that uses
`LoadContext::get_label_handle` to get handles for each `Node` before
populating them. This fixes the problem with Maya rigs, in addition to
making the code simpler and faster.
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