FParseGeoemtry: fix negative offsets

When parsing geometry strings, ensure the "global" screen is taken in to
account.  Prior to this, the individual monitor was assumed.

This should fix GH issue #28.

libs/FScreen.c

@@ -760,21 +760,25 @@ int FScreenParseGeometryWithScreen(
 	int   ret;
 
 	/* Safety net */
-	if (parsestring == NULL || *parsestring == '\0')
+	if (parsestring == NULL || *parsestring == '\0') {
+		*screen_return = NULL;
 		return 0;
+	}
 
-	/* If the geometry specification contains an '@' symbol, assume the
-	 * screen is specified.  This must be the name of the monitor in
-	 * question!
-	 */
+	/* No screen specified; parse geometry as standard. */
 	if (strchr(parsestring, '@') == NULL) {
+		*screen_return = NULL;
 		copy = fxstrdup(parsestring);
 		goto parse_geometry;
 	}
 
+	/* If the geometry specification contains an '@' symbol, assume the
+	 * screen is specified.  This must be the name of the monitor in
+	 * question!
+	 */
 	copy = fxstrdup(parsestring);
 	copy = strsep(&parsestring, "@");
-	*screen_return = parsestring;
+	*screen_return = fxstrdup(parsestring);
 	geom_str = strsep(&copy, "@");
 	copy = geom_str;
 
@@ -800,23 +804,24 @@ int FScreenParseGeometry(
 		parsestring, x_return, y_return, width_return, height_return,
 		&scr);
 
-	if (scr != NULL)
+	if (scr != NULL) {
 		m = monitor_by_name(scr);
-	else
+		free(scr);
+	} else
 		m = monitor_get_current();
 
 	/* adapt geometry to selected screen */
 	if (rc & XValue)
 	{
 		if (rc & XNegative)
-			*x_return -= (m->si->w - m->si->x);
+			*x_return -= (m->virtual_scr.MyDisplayWidth - m->si->w - m->si->x);
 		else
 			*x_return += m->si->x;
 	}
 	if (rc & YValue)
 	{
 		if (rc & YNegative)
-			*y_return -= (m->si->h - m->si->y);
+			*y_return -= (m->virtual_scr.MyDisplayHeight - m->si->h - m->si->y);
 		else
 			*y_return += m->si->y;
 	}