... which constrains an angle to discrete increments in screen
space, similarly to gimp_display_shell_constrain_line().
(cherry picked from commit 8c1a277007)
Fix gimp_constrain_line() and friends to properly constrain line
angles when the image's horizontal and vertical resolutions are
different, and dot-for-dot is disabled.
(cherry picked from commit 4fefab1798)
