terminal: Handle selection for pointer positions outside widget correctly
terminal: Handle selection for pointer positions outside widget correctly

That is, don't crash and select entire first/last line when the pointer
is above or below widget.

jonnylamb/weston,Fantu/compositor-spice,eyolfson/weston,udoprog/weston,Gnurou/weston,xorgy/weston,mchalupa/weston,sir-murray/weston,giucam/weston,sir-murray/weston,kwm81/weston,giucam/weston,mchalupa/weston,jonnylamb/weston,krezovic/weston,udoprog/weston,eyolfson/weston,xorgy/weston,Fantu/compositor-spice,krezovic/weston,Tarnyko/weston-xdg_surface_present,kwm81/weston,Gnurou/weston,Fantu/compositor-spice,Tarnyko/weston-xdg_surface_present