avoid rounding errors when moving knobs or dials using the mouse


git-svn-id: svn://svn.code.sf.net/p/qwt/code/branches/qwt-6.1@1995 29e17f8d-cf37-471e-a288-72cc03a76eda
avoid rounding errors


git-svn-id: svn://svn.code.sf.net/p/qwt/code/branches/qwt-6.1@1999 29e17f8d-cf37-471e-a288-72cc03a76eda