Remove need for GC in Capabilities constructor.

git-svn-id: http://osgworks.googlecode.com/svn/trunk@472 c67418bc-a6eb-11de-8ba5-91705c1537ed
Adding singleton for Capabilities. This will eliminate the need for the app to explicitly create a context and make it current, if the app doesn't want to bother with that.

git-svn-id: http://osgworks.googlecode.com/svn/trunk@473 c67418bc-a6eb-11de-8ba5-91705c1537ed
Make osgwcaps use CapabilitiesSingleton.

git-svn-id: http://osgworks.googlecode.com/svn/trunk@474 c67418bc-a6eb-11de-8ba5-91705c1537ed