app: constrain line angles in display space, not image space
Add an offset_angle parameter to gimp_constrain_line(), which offsets the radial lines by a given angle. Add gimpdisplayshell-utils.[ch], with two new functions: - gimp_display_shell_get_constrained_line_offset_angle(): Returns the offset angle to be passed to gimp_constrain_line(), in order to constrain line angles in display space, according to the shell's rotation angle and flip mode. - gimp_display_shell_constrain_line(): A convenience function which calls gimp_constrain_line() with the said offset angle. Use the new functions in all instances where we constrain line angles, so that angles are constrained in display space, rather than image space. The only exception is GimpEditSelectionTool, which keeps constraining angles in image space, since it's not entirely obvious that we want to constrain angles of dragged layers/selections in display space.
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