Blender Git Loki

- vertex group modifiers isDisabled functions were incorrect, need to check if the string is set: == NULL will never be true.
- was doing NULL checks on freeing memory in cases where the values were already accessed (blender would have crashed anyway), so remove the NULL checks.
- use deform.c api weight functions to replace inline weight lookups in some cases.
- change if checks in weightvg_do_mask() so its more obvious whats going on.