Blender Git Loki

UV: internal changes to picking

Nothing user visible, only things needed for multi-object support,
making picking functions more flexible too.

- Support passing in an initialized hit-struct,
so it's possible to do multiple nearest calls on the same hit data.
- Replace manhattan distance w/ squared distance
so they can be compared.
- Return success to detect changes to a hit-data
which might already be initialized (also more readable).