xwayland: assign view->surface earlier in map handler

xwayland: assign view->surface earlier in map handler

...as it needs to be set before honouring xwayland_surface->fullscreen
because that calls desktop_update_top_layer_visiblity() which relies on
view->surface being set in view_is_focusable() since 13d0b14.

Fix bug introduced by PR #1237 which fails to hide the xfce4-panel (or
any other layer-shell client in the top layer) whilst gaming in
fullscreen. The bug can be observed with the following games:

- Alan Wake 2 (wine)
- Starfield (steam+proton)
- Cyberpunk (steam+proton)
- Quake 1 Remaster (steam-native)

Fixes: #661 (the last bit of it)
Reported-by: @ScarecrowDM
Helped-by: @Consolatis