Merge lp://staging/~vanvugt/mir/fix-1686620 into lp://staging/mir

Ensure fullscreen eglapps that request initial state 'fullscreen'
know their actual buffer dimensions in time to set up glViewport
and render their first frame (LP: #1686620)

Since r4134 they were getting 0x0 from mir_eglapp_init and using
that to set a 0x0 viewport. So the window would remain black until
resized.

Providing resize events asynchronously is fine, but apps also need to know a sane size before that first resize callback, when they're already rendering their first frame.

Sadly that first resize callback is not guaranteed to occur before mir_create_window_sync returns. Even if you move the location of the call introduced in r4150 then it's still racy because we don't have the nice ordering of real wait handles any more. The initial resize event can _still_ occur in a background thread after mir_create_window_sync has returned and the app is setting up its first frame.