Fix incorrect merged weights from ignored bones in FBX import

Fix incorrect merged weights from ignored bones in FBX import

Given a chain of bones A -> B -> C and a vertex weighted to all three
bones: When C is ignored and its weight is to be merged into its parent
B, the sum of the weights that are not A should remain the same as
before the merge. The current code is instead setting the B weight to
the average of the B and C weights, which does not maintain the sum of
the weights. The division that was creating the average has been
removed, so now the weights in C to be merged into B are added directly
to the B weight.

Because the maximum value a weight can be set to is 1.0, this poses a
problem when the weights to be added together exceed 1.0. Re-normalizing
all the weights on the vertex in this case sounds like it could work,
but unfortunately, we only have access to all the weights for one bone
at a time with the current implementation. Re-normalizing the weights
after adding all the weights to each vertex is too late because the
weights exceeding 1.0 would already have been clamped to 1.0. A comment
has been added to indicate that this is a known issue.

Pull Request: https://projects.blender.org/blender/blender-addons/pulls/104928