Core: minor change in SimpleMesh, fixes clipping in regression test
Core: minor change in SimpleMesh, fixes clipping in regression test

* by default a SimpleMesh should not have an index buffer, otherwise it would not be rendered if the index buffer is empty


inviwo/inviwo,inviwo/inviwo,Sparkier/inviwo,Sparkier/inviwo,Sparkier/inviwo,inviwo/inviwo,Sparkier/inviwo,inviwo/inviwo,inviwo/inviwo,inviwo/inviwo,Sparkier/inviwo