window: Fix portrait orientation check for tiling

Use work area from the monitor that the window is currently on to determine if tiling should be allowed.

Window tiling is disabled for monitors with portrait orientation, but the work area we use to detect portrait orientation is taken from the monitor that currently has the mouse pointer.

This works fine for edge tiling using the mouse, but this is broken when using keybindings for window tiling because your mouse pointer could be on a different monitor that has horizontal orientation.