`pen_press` placed each new vertex where it was clicked (after snapping)
whatever the modifiers, so Shift did nothing and a vertical, horizontal or
45° segment had to be lined up by eye. With Shift held, the new vertex now
moves onto the nearest line at a multiple of 45° through the vertex before
it, as far along that line as the click lies (`constrain_45`). Masks and
shape paths both go through this path.