CairoCanvas: implements MixedRoundRect

git-svn-id: http://svn.freepascal.org/svn/lazarus/trunk@40980 4005530d-fff6-0310-9dd1-cebe43e6787f
CairoCanvas: debugclip

git-svn-id: http://svn.freepascal.org/svn/lazarus/trunk@40981 4005530d-fff6-0310-9dd1-cebe43e6787f