fpvectorial: Improves the elliptical arc code

git-svn-id: http://svn.freepascal.org/svn/lazarus/trunk@41561 4005530d-fff6-0310-9dd1-cebe43e6787f
canvas_test: Adds test to arc with start and end point

git-svn-id: http://svn.freepascal.org/svn/lazarus/trunk@41562 4005530d-fff6-0310-9dd1-cebe43e6787f