Wrong "slide out" direction for Top/Bottom-Right possition [Wayland]

Issue Description

On wayland (both Wayfire and Labwc) with position set to "Top right" or "Bottom right", "slide out" direction is not to right but left. Also not slide whole notification, but left side of notification is as screen edge and over this edge is not show sliding notification.

This problem does not occur on X11 session.

Steps to Reproduce Issue

1. Set "Apperance/Default position" to "Top right" or "Bottom right"
2. Run: notify-send --icon="mail-forward" "Bug" "I Want slide to Right :-)"

Expected Behavior

Slide out to right.

Additional Information

1. Confirmed not problem on my setup.
2. For better understanding, screencast: notifyd-wayland-bug-not-slide-to-right.

Versions

• OS distribution and version: ArchLinux
• Overall Xfce version: 4.20
• xfce4-notifyd version (git hash if not a release): 0.9.6-5
• Windowing system (X11/Wayland): Wayland
• Compositor and version (if Wayland): wayfire 0.9.0-1 or labwc 0.8.2-2

Hm, this is probably a result of !70 (merged) (the fix for #132 (closed)). The slide direction will need to be reversed on (only) Wayland when the notifications are on the right half of the screen.