[wayland] Fix Placement::placeTransient keeping screens in bound.

Summary:
If a window flows off the left, we move left of the popup to the left
edge of the screen.

Currently if a window flows off the right, we move the window back by
it's own width, leaving it floating at a random point.
For consistency we should be setting it so the right edge of the popup is on the right
edge of the screen.

So in the auto test for the "right border" case:
The screen is 1280 wide, and we open a 10px popup at 1279  the final X
should be 1270.

Test Plan: Unit test

Reviewers: #kwin, zzag, graesslin

Reviewed By: #kwin, zzag, graesslin

Subscribers: zzag, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D16314