Fixed an off-by-one error in boundX

There was an "off-by-one" error in the boundX function, under the selection.js file.
This means that if you have a selection area of width 800, you could never select the 800th pixel to the right. When you select the whole thing (either by code or by mouse dragging) the whole select area would be highlighted, but the selection message would read: 0px to 799px selected.