Consider a simple client that doesn't use set_window_geometry,
committing after attaching a buffer to a window surface.
Window::geometry falls back to Window::bbox, but when
Shell::map_window called Window::geometry, Window::bbox hadn't been
set yet, because that only happened later in
<State as CompositorHandler>::commit, so the window would be mapped
with a size of (0, 0).
This resulted in server-side decorations not being drawn for such
clients until they were re-focused or resized.
The fix is to add an extra call to Window::on_commit() before mapping
it, so that its bbox is set correctly when Shell::map_window asks for
its geometry.
40eb78debf