more responsive interactive annotation shading threshold for large numbers of sequences

JAL-1326 introduced a routine that updates all the thresholds for all sequence associated annotation being used to shade the alignment.

Currently, all graphlines are updated with each change of the threshold slider, so for large numbers of rows, adjustments may become sluggish. In that case, only the annotation rows currently visible in the display area should be adjusted - until the mouse is released, when all other annotation row thresholds should be updated.