theme: fix button background
1) Added 'no background' if there is only one button and backgrounds
   no more overlap if transparency used.
2) Corrected bug with left button clickable region when window is
   maximized.
Origin: Ubuntu
Bug: https://bugzilla.gnome.org/show_bug.cgi?id=608511
Bug-Ubuntu: https://bugs.launchpad.net/ubuntu/+source/metacity/+bug/535088
Author: Nicolas Desfontaine <noop@free.fr>