intersect canvas rect and painter clip rect for curve polygon clipping


git-svn-id: svn://svn.code.sf.net/p/qwt/code/branches/qwt-6.1-multiaxes@2118 29e17f8d-cf37-471e-a288-72cc03a76eda
compiler warning fixed


git-svn-id: svn://svn.code.sf.net/p/qwt/code/branches/qwt-6.1-multiaxes@2119 29e17f8d-cf37-471e-a288-72cc03a76eda