existing SequenceGroup loses colourscheme when resized in splitframe

When in a splitframe, with no colours applied except to a single group on CDS, resizing the group causes it to lose its colourscheme.

~gmungoc] cause is the SequenceGroup copy constructor (https://source.jalview.org/crucible/browse/~br=develop/jalview/src/jalview/datamodel/SequenceGroup.java?hb=true#to181 ). Resizing triggers MappingUtils.mapSequenceGroup - which has this code:

SequenceGroup mappedGroup = new SequenceGroup(sg);
    mappedGroup.setColourScheme(mapTo.getGlobalColourScheme());
    mappedGroup.clear();

since the new and old group inherit the same shader, both groups' are affected, and the original group get's the alignment background colour.