A lot of applications change the size of the stage from the default
before the stage is initially shown. The size change won't take affect
until the first allocation run. However we want the window to be at
the correct size when we first map it so we should force an allocation
run before showing the stage.

There was an explicit call to XResizeWindow in
clutter_stage_x11_show. This is not needed anymore because
XResizeWindow will already have been called by the allocate method.