Fix rectangle selection edge not follow mouse

Summary:
1. In rectangle mode, when to adjust selection, the right, bottom edge not follow mouse
2. Fix  wrong rectangle size

Test Plan:
1. In rectangle mode, adjust the ( topright, right, bottom right, bottom, bottom left) selection and check mouse and edge position.
2 select full screen and check size
3 save image and check image size

Reviewers: #spectacle, rkflx, ngraham

Reviewed By: #spectacle, rkflx, ngraham

Subscribers: rkflx, ngraham

Differential Revision: https://phabricator.kde.org/D11598