[X11] Fixed border size on Gnome 3

The _NET_FRAME_EXTENTS atom is implemented differently by
different window managers, when window decorations are hidden
with Motif. Unity returns a 0 size, while Gnome 3 returns the
previous size.

This patch removes that ambiguity: when decorations are hidden,
border size becomes zero. This should work everywhere, unless
some window manager decides to troll us by decorating the window
when we explicitly request no decorations. Sigh...