cursor: allow movement until entering the constraint surface

cursor: allow movement until entering the constraint surface

Fixes an issue where the cursor would get stuck (immovable) outside the
window of a Wine/Wayland game, if it was already outside when the game
started (common with a 4:3 game on a 16:9 screen). Now one can manually
move the cursor into the game window, at which point it becomes locked.

This is a minimal/interim fix. Ideally we should warp the cursor into
the constraint area automatically, but that would be a bit more work.

The change to apply_constraint() just turns an assert-failure into a
safe no-op return, since the function is now entered for "locked" as
well as "confined" constraint types.