OpenGL child windows (bug 2398, fixed!)

[denis bonnenfant]

I tested git version with solidworks, and now it's worse than  with
previous chris patch : 

- quite slow, just like with composite patch
- highlighted lines offset, just like with composite patch
- GLXbaddrawable crash when opening multiple windows, regression from
the composite patch, that allows multiple windows 

It looks like all the previous issues are cumulated !

maybe I missed something ?

Denis