don't draw selection too wide (esp visible in full screen mode)

don't draw selection too wide (esp visible in full screen mode)

there is still one issue which is related (or so i think) to the pit is not being updated when extending a selection to a new row with cursor right/left. i left a FIXME in the code

i put this in since it is a uniform improvement on the current situation...

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@24089 a592a061-630c-0410-9148-cb99ea01b6c8