theme: Set -arrow-rise property of backgroundMenu to 0

Background menu shown on right clicking desktop background has an arrow
pointer which points to nothing. This patch sets its height (rise) to 0
so that no arrow is formed.

https://bugzilla.gnome.org/show_bug.cgi?id=699608
boxpointer: Add condition checks for -arrow-rise:0px

This is done for properly drawing popup menu when arrow rise is 0 (in
case of background menu).

Normally, the menu with arrow rise set to 0 is drawn properly having
all four corners rounded. But when the source(click/arrowOrigin) is
near screen's edges, one of the corners (depending on source's position
and arrow alignment) is drawn right angled.

This happens because the rounded corners are skipped and right angled
arrow is drawn when arrow origin is close to the edges.(That's why when
arrow-rise is 0, it forms right angled corner).

So, a few condition checks are made to ensure that right angled corner
is not drawn.

https://bugzilla.gnome.org/show_bug.cgi?id=699608