autodetection of broken raster paint engine


git-svn-id: svn://svn.code.sf.net/p/qwt/code/trunk@2127 29e17f8d-cf37-471e-a288-72cc03a76eda
git-svn-id: svn://svn.code.sf.net/p/qwt/code/trunk@2128 29e17f8d-cf37-471e-a288-72cc03a76eda