Fix line annotation leader line angle

Summary:
For "Straight Line" annotations, the "Leader Lines" (if any) shall be perpendicular to the main line segment. Current code missed to consider page aspect ratio when calculating the main segment angle, which caused non-perpendicular leader lines.

Note: Bug is only visible with non-PDF documents (i.e. where Okular draws line annotation, instead of generator).

Before:
{F6818453}

After:
{F6818452}

Test Plan:
- open a txt file, the generated page will have aspect ratio 3:4
- configure a "Straight Line" annotation with long leader line
- create a line annotation at 45 degree angle
- check the leader line, it shall be perpendicular to the main line

Reviewers: sander

Reviewed By: sander

Subscribers: aacid, okular-devel

Tags: #okular

Differential Revision: https://phabricator.kde.org/D21136