Fix glTF morphing and static skinning

Fix skinning and morphing in glTF for scenes without animation.
Also fix morphing when weights are located on the mesh. See specification:

A Morph Target may also define an optional mesh.weights property that stores the default targets weights. In the absence of a node.weights property, the primitives attributes are resolved using these weights. When this property is missing, the default targets weights are assumed to be zero.