The lines index buffer can contain all edges (edit mode) or only loose
edges (object mode). When switching between these modes the wrong
content of the index buffer can be used.
This patch will clear the lines index buffer when a `loose_edges` is requested. Making sure it is always up to date.
Note that this is supporting an exising hack where the IBO is truncated
during the creation. We should find a different way how to solve these
kind of issues.
Reviewed By: fclem
Differential Revision: https://developer.blender.org/D6349
