openglcontrol: imgui position screen center

git-svn-id: http://svn.freepascal.org/svn/lazarus/trunk@40443 4005530d-fff6-0310-9dd1-cebe43e6787f
openglcontrol: carbon: auxbuffers, multisampling

git-svn-id: http://svn.freepascal.org/svn/lazarus/trunk@40444 4005530d-fff6-0310-9dd1-cebe43e6787f