Blender Git Loki

Collada export: changes to joints/weights in skincontroller

- Collecting joints/weights in one place, it's easier to exclude zero weights or vertexgroups with no matching bone than trying to match same logic in multiple places.
- Still not exporting -1 joints for vertices without weights, but also not outputting -1 joint + weight for each vertexgroup without a matching bone.
- The exported weights are now normalized.

Last I tested this patch stopped 3ds Max crashing on import of file from #29465 (opencollada / internal .dae).