TAChart: Add GUI connector for BGRA drawing back-end

git-svn-id: http://svn.freepascal.org/svn/lazarus/trunk@40178 4005530d-fff6-0310-9dd1-cebe43e6787f
TAChart: Use GUI connector in "bgra" demo

git-svn-id: http://svn.freepascal.org/svn/lazarus/trunk@40179 4005530d-fff6-0310-9dd1-cebe43e6787f