xwm: fix intermittent half-drawn decorations problem

xwm: fix intermittent half-drawn decorations problem

To reproduce the problem:
- start weston (x11 backend worked, glamor in Xwayland makes no
  difference)
- start xterm
- very very slowly move the pointer in the xterm decorations onto or
  away from a button
- the moment the decorations are updated, they will appear incomplete,
  e.g. completely without buttons and title text
- if you cause just one more pointer motion event, the decorations will
  update to completely drawn appearance

Another way to reproduce the problem is to have an xterm and change its
window title. This is easy if you use a shell prompt that updates the
terminal window title. When the title updates, decorations will be
half-drawn until something happens in XWM.

The fix: flush.

Apparently the drawing commands did not get flushed to the X server
until any other X11 action pushed them through.

xcb_flush() is the real fix here. cairo_surface_flush() is added just
for good measure, because documentation indicates it would be better
used, however it was not strictly necessary to fix the problem in my
experiments.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Tested-by: Emmanuel Gil Peyrot <emmanuel.peyrot@collabora.com>
Reviewed-by: Emmanuel Gil Peyrot <emmanuel.peyrot@collabora.com>