incorrect values for NET_WM_STRUT_PARTIAL / NET_WM_STRUT on multi-monitor setup

[Jacob Vlijm]

Plank version: 0.11.4
Window manager: mutter, on Ubuntu Budgie 17.10

On plank, incorrect and in-consequent values are returned for NET_WM_STRUT_PARTIAL / NET_WM_STRUT. When Plank runs on a multi-monitor setup, if plank needs no space on either which of left/right/top/bottom, zero is (correctly) returned, and correctly relative to the current monitor. In case the value is *not* zero however, the returned value is relative to the whole spanning (multi) monitor size, which seems incorrect. Also the window manager seems to get confused therefore, not allowing windows to be positioned on available space.