Add 8-bone support for MeshDataTool

[gongpha]

The current MeshDataTool supports only 4 bones. Making 8-bone mesh corrupted when committing

Reference :

var mdt := MeshDataTool.new()
mdt.create_from_surface(meshinstance3d.mesh, 0)
var m := ArrayMesh.new()
mdt.commit_to_surface(m)
meshinstance3d.mesh = m

Before	After

[gongpha]

I'm not sure if ERR_* inside the if clause is a decent code style. I didn't see it used much elsewhere

[AThousandShips]

In this case it's good (clearer than the confusing message if combining the two), but these should be given a descriptive error message to clarify why it's four or eight bones, in case someone intends to use 8 but doesn't know to set the flag, or forgets to not set the flag