fix: xmodels with more than one root bone are not linkable

[Laupetin]

Apparently I thought there were no models with more than one root bone, but there are.
e.g.

• IW3 code_post_gfx void
• T6 common_mp t6_wpn_smg_pdw57_view

and probably a bunch more.

TODOs:

• Properly add all root bones to dumped models skin
• Load bones and child bones for all root bones and don't error on more than one root bone
• Reorder bones if they are not sorted by parent (they are exported as such, but this order is not guaranteed to come from external tools)

fixes: #733