QwtAlphaColorMap fixes


git-svn-id: svn://svn.code.sf.net/p/qwt/code/branches/qwt-6.1@2085 29e17f8d-cf37-471e-a288-72cc03a76eda
spectrogram showing different color maps


git-svn-id: svn://svn.code.sf.net/p/qwt/code/branches/qwt-6.1@2086 29e17f8d-cf37-471e-a288-72cc03a76eda
drawing of color bars with semi transparent colors fixed


git-svn-id: svn://svn.code.sf.net/p/qwt/code/branches/qwt-6.1@2090 29e17f8d-cf37-471e-a288-72cc03a76eda