xwayland: set initial geometry in map_request handler

xwayland: set initial geometry in map_request handler

Set the initial geometry of maximized/fullscreen views before
actually mapping them, so that they can do their initial layout and
drawing with the correct geometry. This avoids visual glitches and
also avoids undesired layout changes with some apps (e.g. HomeBank).

Fixes: #1320
v2: ensure valid geometry for unmanaged->managed case