Fullscreen windows should get pointer confinement activated immediately by default

Currently, when pointer confinement is asking for a window, user has to click on the window for the confinement to actually happen. That's a compositor policy, implemented on maybe_enable_pointer_constraint() here: https://gitlab.freedesktop.org/wayland/weston/-/blob/main/libweston/input.c#L3729

However, in the case of fullscreen windows, they should have the pointer confinement granted. Think of any fullscreen UI or game where the pointer is only supposed to move on a part of the screen: it's very strange to have users click on screen before pointer confinement happens in that case.

So, modification of maybe_enable_pointer_constraint() is needed to detect whether a window is fullscreen, and grant pointer confinement immediately in that case.