[client] cursor: restore Wayland edge synchronization

Keep guest-to-host pointer synchronization active while an attempted
surface exit is blocked by the edge of a Wayland output. Continue
forwarding relative motion and wait for wl_pointer.leave before marking
the cursor outside the window.

Distinguish whole-surface releases from exits into letterbox space so
the expected asynchronous release callback cannot disable the feedback
loop before the pointer is confined again.

Treat Wayland pointer capability loss as a real leave so an unrelated
inactive callback cannot be mistaken for the intentional release.

client/src/main.c

@@ -147,6 +147,7 @@ static void lgInit(void)
   g_cursor.inView        = false;
   g_cursor.viewReq       = false;
   g_cursor.exit          = false;
+  g_cursor.surfaceExit   = false;
   g_cursor.guest.valid   = false;
 
   // if spice is not in use, hide the local cursor