take an empty region as an indication to remove the clipping, but on R5,
this is actually valid... this patch fixes the problem
* the ViewState::clipping_region is now consistently used to memorize
the user defined clipping of the view state instead of being sometimes
used to cache the current view clipping
git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@16656 a95241bf-73f2-0310-859d-f6bbb57e9c96