Adjust selection point coords when scrollback shrinks
requested to merge ninjalj/konsole:work/ninjalj/adjust-selection-point-on-scrollback-shrink into master
The selection point coordinates are relative to the start of the scrollback, so when the scrollback shrinks, either because it is limited to some amount of lines, or because it is cleared, we could end up with stale coordinates which could cause a crash on a subsequent selection extend operation.