erratic column selection when quickly dragging across alignment ruler

Noticed when testing fix for JAL-2151. Raised as defect for 2.10.0

1. Open Feredoxin applet example, and hide columns 1-5, then columns 10-15.
2. Drag to select a range of columns from column 7 through to 25.

* no hidden columns
- click-drag to right smoothly extends a selection which smoothly shrinks as drag back to left, eventually moving left hand border rather than right.

* hidden columns
- click drag at slow speed from column 7 to right extends selection. dragging back to left leaves first 5 visible columns selected, but rubber banded sequence group shrinks to 2 columns. but no more.
- click drag at high speed to right beginning at any position leaves at least one or two column not selected.

looks like there are still bugs in the stretchgroup code where hidden columns are not being accounted for.

Tested on firefox: Java Plug-in 11.101.2.13
Using JRE version 1.8.0_101-b13 Java HotSpot(TM) 64-Bit Server VM