When we resize, we relied on the stage's allocate to re-initialise the
GL viewport. Unfortunately, if we resized within Clutter, the new size
was cached before the window is actually resized, so glViewport wasn't
being called after resizing (some of the time, it's a race condition).

Change the way resizing works slightly so that we only resize when the
geometry size doesn't match our preferred size, and queue a relayout on
ConfigureNotify so the glViewport gets called.

Also change window creation slightly so that setting the size of a
window before it's realized works correctly.