Blender Git Loki

Fix T35993, issue with viewport when using instances.

Not really sure what the issue here was initially (might have been a
driver problem) but looks like the issue is solved if we skip generating
a display list for clipped objects; Driver does not like a generated
list that is not drawn.

Now we try to clip the object always, not only when using display lists.
This means that display lists will only get generated if object is
actually visible.
Also used glPush/glPop for instance matrices since glLoad can cause
recalculation of inverses in the driver.