[x11] Don't set actor size on ConfigureNotify

[x11] Don't set actor size on ConfigureNotify

Calling clutter_actor_set_size in response to ConfigureNotify makes
setting the size of the stage racy - the most common result of which
seems to be that you can't set the stage dimensions to anything less
than 640x480.

Instead, add a first_allocation bit to the private structure of the X11
stage and force the first resize (necessary or the default stage will be
a 1x1 window).