added implementation of gtk3 polygon

git-svn-id: http://svn.freepascal.org/svn/lazarus/trunk@43698 4005530d-fff6-0310-9dd1-cebe43e6787f
gtk3 added note about wrong drawing position inside tpage. added case for gdk_unmap

git-svn-id: http://svn.freepascal.org/svn/lazarus/trunk@43699 4005530d-fff6-0310-9dd1-cebe43e6787f