TAChart: Support PutImage in BGRA drawing back-end

git-svn-id: http://svn.freepascal.org/svn/lazarus/trunk@40081 4005530d-fff6-0310-9dd1-cebe43e6787f
TAChart: Minor refactoring of "bgra" demo

git-svn-id: http://svn.freepascal.org/svn/lazarus/trunk@40082 4005530d-fff6-0310-9dd1-cebe43e6787f