tweak colorRamp for (optional) non-opaque colours

git-svn-id: https://svn.r-project.org/R/trunk@63408 00db46b3-68df-0310-9c12-caf00c1e9a41
trap colours with alpha values earlier

git-svn-id: https://svn.r-project.org/R/trunk@63409 00db46b3-68df-0310-9c12-caf00c1e9a41