This PR fixes #845 and adds some QoL improvements on the process of weights.
Vertex groups for each vertex are sorted by weights. Before the patch the default behavior simply ignores any group with an index greater than 3 and will be likely to accidentially prevent some groups with significant influence from exporting when there are lots of groups per vertex.
get_bone_by_vgroup() now uses -1 for the vertex group without a corresponding bone. Before the patch it uses 0 but there is always an actual bone with 0 index (e.g. SKEL_Root).
This PR fixes #845 and adds some QoL improvements on the process of weights.