TAChart: Add TABGRAUtils unit

git-svn-id: http://svn.freepascal.org/svn/lazarus/trunk@40170 4005530d-fff6-0310-9dd1-cebe43e6787f
TAChart: Use TABGRAUtils unit in "bgra" demo

git-svn-id: http://svn.freepascal.org/svn/lazarus/trunk@40171 4005530d-fff6-0310-9dd1-cebe43e6787f