Correct way for reserved space on multihead environment.

Applications such a panels or docks have ability for reserving space so
that no windows should cover or overlap them. There is (partially)
support for it by checking the _NET_WM_STRUT property, although
calculation for the reserved space was wrong in case of several heads
enabled.

In this patch calculation for reserved area has been corrected.

src/wmspec.c

@@ -845,10 +845,43 @@ Bool wNETWMGetUsableArea(WScreen *scr, int head, WArea *area)
 
 	rect = wGetRectForHead(scr, head);
 
-	area->x1 = rect.pos.x + area->x1;
+    /* NOTE(gryf): calculation for the reserved area should be preformed for 
-	area->x2 = rect.pos.x + rect.size.width - area->x2;
+     * current head, but area, which comes form _NET_WM_STRUT, has to be 
-	area->y1 = rect.pos.y + area->y1;
+     * calculated relative to entire screen size, i.e. suppose we have three 
-	area->y2 = rect.pos.y + rect.size.height - area->y2;
+     * heads arranged like this:
+     *
+     * ╔════════════════════════╗
+     * ║ 0                      ║
+     * ║                        ╠═══════════╗
+     * ║                        ║ 2         ║
+     * ║                        ║           ║
+     * ╟────────────────────────╫───────────╢
+     * ╠════════════════════════╬═══════════╝
+     * ║ 1                      ║
+     * ║                        ║
+     * ║                        ║
+     * ║                        ║
+     * ║                        ║
+     * ╟────────────────────────╢
+     * ╚════════════════════════╝
+     *
+     * where head 0 have resolution 1920x1080, head 1: 1920x1200 and head 2 
+     * 800x600, so the screen size in this arrangement will be 2720x2280 (1920 
+     * + 800) x (1080 + 1200).
+     *
+     * Bottom line represents some 3rd party panel, which sets properties in 
+     * _NET_WM_STRUT_PARTIAL and optionally _NET_WM_STRUT.
+     *
+     * By coincidence, coordinates x1 and y1 from left and top are the same as 
+     * the original data which came from _NET_WM_STRUT, since they meaning 
+     * distance from the edge.
+     */
+
+    /* optional reserved space from right */
+    area->x2 = scr->scr_width - area->x2;
+
+    /* optional reserved space from bottom */
+	area->y2 = scr->scr_height - area->y2;
 
 	return True;
 }