New view manager enabled by default

[Robert O'Callahan]

The new view manager has been enabled by default. You 
can disable it by setting

 
user_pref("nglayout.debug.enable_scary_view_manager", 
true);

in your prefs.js file. So if you see new bugs that may 
be view manager-related, please try them with the old 
view manager.

This checkin fixes a lot of bugs, especially bugs 
related to combinations of fixed positioning, 
transparency/translucency, clipping, and z-index 
handling. Discussion is in
http://bugzilla.mozilla.org/show_bug.cgi?id=39621

Rob